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| [[File:Copyleft.svg|thumb|alt=Small letter c turned 180 degrees, surrounded by a single line forming a circle.|Copyleft symbol]]
| | http://en.wikipedia.org/wiki/Copyleft |
| '''Copyleft''' is a [[Word play|play on the word]] ''[[copyright]]'' to describe the practice of using copyright law to offer the right to distribute copies and modified versions of a work and '''requiring''' that the same rights be preserved in modified versions of the work. In other words, copyleft is a general method for making a program (or other work) free ([[Gratis_versus_libre|''libre'']]), and requiring all modified and extended versions of the program to be free as well.<ref>{{cite web|url=http://www.gnu.org/copyleft |title=What is Copyleft? |accessdate=2010-08-29}}</ref>
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| Copyleft is a form of [[License|licensing]] and can be used to maintain copyright conditions for works such as [[computer software]], [[documents]], and [[art]]. In general, copyright law is used by an author to prohibit others from reproducing, adapting, or distributing copies of the author's work. In contrast, under copyleft, an author may give every person who receives a copy of a work permission to reproduce, [[Derivative work|adapt]] or distribute it and require that any resulting copies or adaptations are also bound by the same licensing agreement.
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| Copyleft licenses (for software) require that information necessary for reproducing and modifying the work must be made available to recipients of the [[executable]]. The [[source code]] files will usually contain a copy of the license terms and acknowledge the author(s).
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| Copyleft type licenses are a novel use of existing copyright law to ensure a work remains freely available. The [[GNU General Public License]], originally written by [[Richard Stallman]], was the first copyleft license to see extensive use, and continues to dominate the licensing of copylefted software. [[Creative Commons]], a [[non-profit organization]] founded by [[Lawrence Lessig]], provides a similar license condition called [[ShareAlike]].
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| == Reciprocity ==
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| Copyleft can be characterized as a copyright licensing scheme in which an author surrenders some but not all rights under copyright law. Instead of allowing a work to fall completely into the [[public domain]] (where no ownership of copyright is claimed), copyleft allows an author to impose some restrictions on those who want to engage in activities that would more usually be reserved by the copyright holder. Under copyleft, derived works may be produced provided they are released under the compatible copyleft scheme.
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| The underlying principle is that one benefits freely from the work of others but any modifications one makes must be released under compatible terms. For this reason some copyleft licenses are also known as [[reciprocity (social psychology)|reciprocal]] licenses, they have also been described as "[[wikt:viral|viral]]" due to their self-perpetuating terms.<ref>{{cite web
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| |url=http://www.microsoft.com/presspass/exec/craig/05-03sharedsource.mspx
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| |title=Prepared Text of Remarks by Craig Mundie, Microsoft Senior Vice President - The Commercial Software Model
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| |date=May 3, 2001
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| |publisher=[[New York University Stern School of Business]]
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| |first=Craig
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| |last=Mundie
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| |authorlink=Craig Mundie
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| |accessdate=2009-10-01
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| }}</ref>
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| Under [[fair use]], however, the copyleft license may be superseded, just like regular copyrights. Therefore, any person utilizing a copyleft-licensed source for their own work is free to choose any other license provided they meet the fair use standard.<ref>
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| {{Cite book
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| | author = Brian Still & Kirk St. Amant
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| | chapter = Examining Open Source Software Licenses through the Creative Commons Licensing Model
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| | title = Handbook of Research on Open Source Software: Technological, Economic, and Social Perspectives
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| | publisher = Information Science Reference
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| | date = 2007
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| | pages = 382 of 728
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| | isbn = 1-59140-999-3}}</ref>
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| While copyright law gives software ''authors'' control over copying, distribution and modification of their works, the goal of copyleft is to give '''all''' ''users'' of the software the [[Gratis versus Libre|freedom]] to carry out these activities. In this way, copyleft licenses are distinct from other types of [[free software licenses]], which do not guarantee that all "downstream" recipients of the program receive these rights, or the [[source code]] needed to make them effective. In particular, [[permissive free software licence|permissive]] free software licenses such as [[BSD licenses|BSD]] allow re-distributors to remove some or all these rights, and do not require the distribution of source code.
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| ==History==
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| [[File:Copyleft All Wrongs Reserved.png|thumb|240px|right|alt=Monospaced font reads "Tiny basic for Intel 8080, version 2.0 by Li-Chen Wang, modified and translated to Intel mnemonics by Roger Rausklob, 10 October 1976. @ Copyleft, All Wrongs Reserved."|The use of "Copyleft; All Wrongs Reserved" in 1976]]
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| An early example of copyleft was the [[Tiny BASIC]] project started in the newsletter of the People's Computer Company in 1975. Dennis Allison wrote a specification for a simple version of the BASIC programming language.<ref name = "Allison 1976">{{Cite journal| last = Allison | first = Dennis | title = Design notes for TINY BASIC | journal = SIGPLAN Notices | volume = 11 | issue = 7 | pages = pp. 25–33 | publisher = ACM |date = July 1976| doi = 10.1145/987491.987494 }} The [[Association for Computing Machinery|ACM]] Special Interest Group on Programming Languages (SIGPLAN) reprinted the Tiny Basic design notes from the January 1976 Tiny BASIC Journal.</ref> This design did not support text strings and only used integer arithmetic. The goal was for the program to fit in 2 to 3 [[kilobyte]]s of memory.
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| The Tiny BASIC contents of the newsletter soon became ''[[Dr. Dobb's Journal]] of Tiny BASIC'' with a subtitle of "Calisthenics & Orthodontia, Running Light Without Overbyte." Hobbyists began writing BASIC language interpreters for their microprocessor-based home computers and sending the source code to ''Dr. Dobb's Journal'' and other magazines to be published. By the middle of 1976, Tiny BASIC interpreters were available for the [[Intel 8080]], the [[Motorola 6800]] and [[MOS Technology 6502]] processors. This was a free software project before the internet allowed easy transfer of files. Computer hobbyists would exchange paper tapes, cassettes or even retype the files from the printed listings.<ref name = "Warren 1976">{{Cite journal| last = Warren | first = Jim C. | authorlink =Jim Warren (computer specialist)| title = Correspondence | journal = SIGPLAN Notices | volume = 11 | issue = 7 | pages = pp. 1–2 | publisher = ACM | issn = 0362-1340 |date=July 1976}}</ref>
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| Jim Warren, editor of ''Dr. Dobb's Journal'', wrote in the July 1976 [[Association for Computing Machinery|ACM]] Programming Language newsletter about the motivations and methods of this successful project. He started with this: "There is a viable alternative to the problems raised by Bill Gates in his irate [[Open Letter to Hobbyists|letter to computer hobbyists]] concerning 'ripping off' software. When software is free, or so inexpensive that it's easier to pay for it than to duplicate it, then it won't be 'stolen'." The method was to have an experienced professional do the overall design and then outline an implementation strategy. Knowledgeable amateurs would implement the design for a variety of computer systems. Warren predicted this strategy would be continued and expanded.<ref name="Warren 1976"/>
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| The May 1976 issue of ''Dr. Dobbs Journal'' had [[Li-Chen Wang]]'s Palo Alto Tiny BASIC for the Intel 8080 microprocessor. The listing began with the usual title, author's name and date but it also had "@COPYLEFT ALL WRONGS RESERVED".<ref>{{Cite journal|last=Wang |first=Li-Chen |authorlink=Li-Chen Wang |title=Palo Alto Tiny BASIC |journal=[[Dr. Dobb's Journal|Dr. Dobb's Journal of Computer Calisthenics & Orthodontia, Running Light Without Overbyte]] |volume=1 |issue=5 |pages=12–25 |month=May |year=1976}} Source code begins with the following six lines. "TINY BASIC FOR INTEL 8080; VERSION 1.0; BY LI-CHEN WANG; 10 JUNE, 1976; @COPYLEFT; ALL WRONGS RESERVED" The June date in the May issue is correct. The magazine was behind schedule, the June and July issues were combined to catch up.</ref> A fellow [[Homebrew Computer Club]] member, Roger Rauskolb, modified and improved Li-Chen Wang's program and this was published in the December 1976 issue of ''Interface Age'' magazine.<ref>{{Cite journal|last=Rauskolb |first=Roger |title=Dr. Wang's Palo Alto Tiny BASIC |journal=Interface Age |volume=2 |issue=1 |pages=92–108 |month=December |year=1976}} The source code begins with the following nine lines: <tt>TINY BASIC FOR INTEL 8080; VERSION 2.0; BY LI-CHEN WANG; MODIFIED AND TRANSLATED TO INTEL MNEMONICS; BY ROGER RAUSKOLB; 10 OCTOBER, 1976 ; @COPYLEFT; ALL WRONGS RESERVED</tt></ref> Roger added his name and preserved the COPYLEFT Notice.
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| A later instance of copyleft arose when [[Richard Stallman]] was working on a [[Lisp programming language|Lisp]] interpreter. [[Symbolics]] asked to use the Lisp interpreter, and Stallman agreed to supply them with a public domain version of his work. Symbolics extended and improved the Lisp interpreter, but when Stallman wanted access to the improvements that Symbolics had made to his interpreter, Symbolics refused. Stallman then, in 1984, proceeded to work towards eradicating this emerging behavior and culture of [[proprietary software]], which he named ''software hoarding''. This wasn't the first time Stallman had dealt with proprietary software but he deemed this interaction as a 'turning point.' He justified software sharing protesting that when sharing, the software online can be copied without the loss of the original piece of work. Everyone's a winner. The software can be used multiple times without ever being damaged or wearing out.<ref>{{cite web|last=Carver|first=Brian W.|title=Share and Share Alike: Understanding and Enforcing Open Source and Free Software Licenses|url=http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1586574|publisher=Berkeley Technology Law Journal|accessdate=2/6/12}}</ref> <ref>{{Cite book|url=http://www.oreilly.com/openbook/freedom |title=Free as in Freedom - Richard Stallman's Crusade for Free Software |first=Sam |last=Williams |authorlink=Sam Williams (American journalist) |month=March |year=2002 |chapter=7 |isbn=0-596-00287-4 |publisher=[[O'Reilly Media]] |chapterurl=http://www.oreilly.com/openbook/freedom/ch07.html}}</ref>
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| As Stallman deemed it impractical in the short term to eliminate current copyright law and the wrongs he perceived it perpetuated, he decided to work within the framework of existing [[law]]; In 1988, he created his own copyright license, the Emacs General Public License,<ref>{{cite web|url=http://www.free-soft.org/gpl_history/emacs_gpl.html |title=Emacs General Public License
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| |date=2001-07-05 |accessdate=2008-08-23}}</ref> the first copyleft license. This later evolved into the [[GNU General Public License]], which is now one of the most popular Free Software licenses. For the first time a copyright holder had taken steps to ensure that the maximal number of rights be perpetually transferred to a program's users, no matter what subsequent revisions anyone made to the original program. This original GPL did not grant rights to the public at large, only those who had already received the program; but it was the best that could be done under existing law.
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| The new license was not at this time given the copyleft label.<ref name="autogenerated1">{{cite web|url=http://www.gnu.org/gnu/thegnuproject.html |title=About the GNU Project |publisher=[[Free Software Foundation]] |first=Richard |last=Stallman |authorlink=Richard Stallman |date=2008-01-21 |accessdate=2008-08-23}}</ref> Richard Stallman stated that the use of "Copyleft" comes from [[Don Hopkins]], who mailed him a letter in 1984 or 1985 on which was written: "Copyleft—all rights reversed."<ref name="autogenerated1" /> The term "kopyleft" with the notation "[[All Rites Reversed]]" was also in use in the early 1970s within the [[Principia Discordia]], which may have inspired Hopkins or influenced other usage. And in the arts [[Ray Johnson]] had earlier coined the term independently as it pertained to his making of and distribution of his mixed media imagery in his [[mail art]] and ephemeral gifts, for which he encouraged the making of [[derivative work]]s. (While the phrase itself appears briefly as (or on) one of his pieces in the 2002 documentary ''[[Ray Johnson|How to Draw a Bunny]]'', Johnson himself is not referenced in the 2001 documentary ''[[Revolution OS]]''.)
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| Some{{Who|date=June 2012}} have suggested that copyleft became a divisive issue in the ideological strife between the [[Open Source Initiative]] and the [[free software movement]].<ref>{{cite web|url=http://www.onlamp.com/pub/a/onlamp/2005/06/30/esr_interview.html |title=ESR: "We Don't Need the GPL Anymore" |date=2005-06-30 |publisher=ONLamp.com |first=Federico |last=Biancuzzi |accessdate=2008-08-23}}</ref> However, there is evidence that copyleft is both accepted and proposed by both parties:
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| * Both the OSI and the FSF have copyleft and non-copyleft licenses in their respective lists of accepted licenses.<ref name="osi">{{cite web|url=http://www.opensource.org/licenses/alphabetical |title=Licenses by Name | first=Michael |last=Tiemann |authorlink=Michael Tiemann |date=2006-09-18 |accessdate=2008-08-23}}</ref><ref name="fsf">{{cite web|url=http://www.fsf.org/licensing/licenses |title=Licenses |date=2008-06-05 |accessdate=2008-08-23}}</ref>
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| * The OSI's original Legal Counsel [[Lawrence Rosen (attorney)|Lawrence Rosen]] has written a copyleft license, the [[Open Software License]].
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| * The OSI's licensing how-to recognises the GPL as a "best practice" license.<ref>{{cite web|url=http://www.catb.org/~esr/Licensing-HOWTO.html#choosing |title=Licensing HOWTO |date=2002-11-09 |first=Eric Steven |last=Raymond |authorlink=Eric S. Raymond |accessdate=2008-08-23}}</ref>
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| * Some of the software programs of the GNU Project are published under non-copyleft licenses.<ref>[http://yro.slashdot.org/comments.pl?sid=225606&cid=18272276 What the GPLv3 Means for MS-Novell Agreement<!-- Bot generated title -->]</ref>
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| * Stallman himself has endorsed the use of non-copyleft licenses in certain circumstances, most recently in the case of the [[Ogg Vorbis]] license change.<ref>{{cite web|url=http://lwn.net/2001/0301/a/rms-ov-license.php3 |title=LWN.net: RMS on the Ogg Vorbis license |date=2001-02-26 |first=Richard |last=Stallman |authorlink=Richard Stallman |quote=[M]y agreement with the idea of a lax [Ogg/Vorbis] license in this special case is just as pragmatic as my preference for the GPL in most cases. In both cases it is a matter of how we can attain freedom. |accessdate=2008-08-23}}</ref>
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| ==Applying copyleft==
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| Common practice for using copyleft is to codify the copying terms for a work with a [[license]]. Any such license typically gives each person possessing a copy of the work the same freedoms as the author, including (from the [[Free Software Definition]]):
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| : Freedom 0 – the freedom to use the work,
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| : Freedom 1 – the freedom to study the work,
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| : Freedom 2 – the freedom to copy and share the work with others,
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| : Freedom 3 – the freedom to modify the work, and the freedom to distribute modified and therefore derivative works.
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| (Note that the list begins from 0 due to [[zero-based indexing|a coding tradition]]—the first array element in [[C (programming language)|C]] and many other programming languages is numbered as 0.)
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| These freedoms do not ensure that a [[derivative work]] will be distributed under the same liberal terms. In order for the work to be truly copyleft, the license has to [[Obligation|ensure]] that the author of a derived work can only distribute such works under the same or equivalent license.
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| In addition to restrictions on copying, copyleft licenses address other possible impediments. These include ensuring the rights cannot be later [[revoke]]d and requiring the work and its derivatives to be provided in a form that facilitates modification. In [[Computer software|software]], this requires that the [[source code]] of the derived work be made available together with the software itself.
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| Copyleft licenses necessarily make creative use of relevant rules and laws. For example, when using copyright law, those who contribute to a work under copyleft usually must gain, defer or assign copyright holder status. By submitting the copyright of their contributions under a copyleft license, they deliberately give up some of the rights that normally follow from copyright, including the right to be the unique distributor of copies of the work.
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| Some laws used for copyleft licenses vary from one country to another, and may also be granted in terms that vary from country to country. For example, in some countries it is acceptable to sell a software product without warranty, in standard GNU GPL style (see articles 11 and 12 of [http://www.fsf.org/licenses/gpl.html the GNU GPL version 2]), while in most [[Europe]]an countries it is not permitted for a software distributor to [[waiver|waive]] all warranties regarding a sold product. For this reason the extent of such warranties are specified in most European copyleft licenses. Regarding that, see the European Union Public Licence [[EUPL]],<ref>{{cite web|url=http://www.osor.eu/eupl/european-union-public-licence-eupl-v.1.1 |title=The EUPL - European Union Public Licence |publisher=European Commission |accessdate=2007-01-09}}</ref> or the [[CeCILL license]],<ref>{{cite web|url=http://www.inria.fr/valorisation/logiciels/Licence.CeCILL-V1.US.pdf |title=Free Software Licensing Agreement CeCILL |publisher=INRIA |accessdate=2010-08-24}}</ref> a license that allows one to use GNU GPL (see article 5 of the EUPL and article 5.3.4 of CeCILL) in combination with a limited warranty (see article 7 and 8 of the EUPL and 9 of CeCILL).
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| ==Types of copyleft and relation to other licenses==
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| {{See also|Free software licences#Restrictions}}
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| Copyleft is a distinguishing feature of some [[free software]] licenses. Many [[free software licenses]] are not copyleft licenses because they do not require the licensee to distribute derivative works under the same license. There is an ongoing debate as to which class of license provides the greater degree of freedom. This debate hinges on complex issues such as the definition of [[Political freedom|freedom]] and whose freedoms are more important, or whether to maximize the freedom of all potential future recipients of a work (''freedom from'' the creation of proprietary software). Non-copyleft free software licenses maximize the freedom of the initial recipient (''freedom to'' create proprietary software).
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| In common with the [[Creative Commons]] share-alike licensing system, GNU's Free Documentation License allows authors to apply limitations to certain sections of their work, exempting some parts of their creation from the full copyleft mechanism. In the case of the GFDL, these limitations include the use of invariant sections, which may not be altered by future editors. The initial intention of the GFDL was as a device for supporting the [[documentation]] of copylefted software. However, the result is that it can be used for any kind of document.
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| ===Strong and weak copyleft===
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| The strength of the copyleft governing a work is an expression of the extent that the copyleft provisions can be efficiently imposed on all kinds of derived works. "Weak copyleft" refers to licenses where not all derived works inherit the copyleft license; whether a derived work inherits or not often depends on the manner in which it was derived.
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| "Weak copyleft" licenses are generally used for the creation of [[Library (computing)|software libraries]], to allow other software to link to the library, and then be redistributed without the legal requirement for the work to be distributed under the library's copyleft license. Only changes to the weak copylefted software itself become subject to the copyleft provisions of such a license, not changes to the software that links to it. This allows programs of any license to be compiled and linked against copylefted libraries such as [[GNU C Library|glibc]] (the [[GNU project]]'s implementation of the [[C standard library]]), and then redistributed without any re-licensing required.
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| The most well known free software license that uses strong copyleft is the GNU General Public License. Free software licenses that use "weak" copyleft include the [[GNU Lesser General Public License]] and the [[Mozilla Public License]]. Examples of non-copyleft free software licenses include the [[MIT License|X11 license]], [[Apache license]] and the [[BSD licenses]].
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| The [[Design Science License]] is a strong copyleft license that can apply to any work that is not software or documentation, such as art, music, sports photography, and video. It is hosted on the Free Software Foundation website's [http://www.gnu.org/licenses/license-list.html license list], but it is not considered compatible with the GPL by the Free Software Foundation.
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| ===Full and partial copyleft===
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| "Full" and "partial" copyleft relate to another issue: Full copyleft exists when all parts of a work (except the license itself) may only be modified and distributed under the terms of the work's copyleft license. Partial copyleft exempts some parts of the work from the copyleft provisions, thus permitting distribution of some modifications under terms other than the copyleft license, or in some other way does not impose all the principles of copylefting on the work. For example, the GPL linking exception made for some software packages (see below).
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| ===Share-alike===
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| [[Share-alike]] imposes the requirement that any freedom that is granted regarding the original work must be granted on exactly the same or compatible terms in any derived work: this implies that any copyleft license is automatically a share-alike license, but not the other way around, as some share-alike licenses include further restrictions, for instance prohibiting commercial use. Another restriction is that not everyone wants to share their work and some share-alike agreements require that the whole body of work be shared, even if the author only wants to share a certain part. The plus side for the author of the source code is that any modification to the code will not only benefit the company, but the author will be recognized and hold equal claim over the changed code.<ref>{{cite web|last=Engelfriet|first=Arnoud|title=Choosing an Open Source License|url=http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5370763|publisher=Athens University of Economics and Business|accessdate=2/6/12}}</ref> <ref>Rob Myers, [http://www.robmyers.org/weblog/2008/02/24/noncommercial-sharealike-is-not-copyleft/ NonCommercial Sharealike is not Copyleft]</ref> Some permutations of the [[Creative Commons licenses]] are examples of share-alike.
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| ==Viral licensing==
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| {{Main|Viral license}}
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| Copyleft licenses are sometimes referred to as "[[viral license]]s" because any works derived from a copyleft work must themselves be copyleft when distributed (and thus they exhibit a [[viral phenomenon]]). The term 'General Public Virus', or 'GNU Public Virus' (GPV), has a long history on the Internet, dating back to shortly after the GPL was first conceived.<ref>{{cite web|url=http://psg.com/lists/namedroppers/namedroppers.2006/msg00246.html |title=Re: Section 5.2 (IPR encumberance) in TAK rollover requirement draft |first=Paul |last=Vixie |authorlink=Paul Vixie |publisher=[[Internet Engineering Task Force|IETF]] Namedroppers mailing list |date=2006-03-06 |accessdate=2007-04-29 |archiveurl = http://web.archive.org/web/20070927175628/http://psg.com/lists/namedroppers/namedroppers.2006/msg00246.html |archivedate = 2007-09-27}}</ref><ref>{{cite web|url=http://catb.org/esr/jargon/oldversions/jarg221.txt |title=General Public Virus |work=[[Jargon File]] 2.2.1 |date=1990-12-15 |accessdate=2007-04-29}}</ref><ref>{{Cite journal|url=http://devlinux.org/lw-gnu-published.html |title=Reverse-engineering the GNU Public Virus — Is copyleft too much of a good thing? |first=Stig |last=Hackvän |journal=[[Linux Journal]]|month=September |year=1999 |accessdate=2007-04-29}}</ref> Many [[BSD Licenses|BSD License]] advocates used the term derisively<ref>{{cite web|url=http://cypherpunks.venona.com/date/1998/10/msg00429.html |title=Re: propose: `cypherpunks license' (Re: Wanted: Twofish source code) |first=Bill |last=Stewart |publisher=[[Cypherpunk]]s mailing list |date=1998-10-08 |accessdate=2007-04-29}}</ref><ref>{{cite web|url=http://gcc.gnu.org/ml/gcc/2000-10/msg00198.html |title=Re: Using of parse tree externally |first=Joe |last=Buck |publisher=[[GNU Compiler Collection|GCC]] mailing list|date=2000-10-10 |accessdate=2007-04-29}}</ref><ref>{{cite web|url=http://themes.freshmeat.net/articles/view/172#comment-5548 |title=The GNU Public Virus |first=L. Adrian |last=Griffis |date=2000-07-15 |accessdate=2007-04-29}}</ref> in regards to the GPL's tendency to absorb BSD licensed code without allowing the original BSD work to benefit from it, while at the same time promoting itself as "freer" than other licenses. [[Microsoft]] vice-president [[Craig Mundie]] remarked "This viral aspect of the GPL poses a threat to the intellectual property of any organization making use of it."<ref>{{cite web|url=http://www.microsoft.com/presspass/exec/craig/05-03sharedsource.mspx |title=Speech Transcript - Craig Mundie |publisher=[[New York University Stern School of Business]] |date=2001-05-03 |first=Craig |last=Mundie |authorlink=Craig Mundie |accessdate=2008-08-23}}</ref> In another context, [[Steve Ballmer]] declared that code released under GPL is useless to the commercial sector (since it can only be used if the resulting surrounding code becomes GPL), describing it thus as "a cancer that attaches itself in an intellectual property sense to everything it touches".<ref>{{Cite news
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| | first=Dave
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| | last=Newbart
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| | pages=
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| | title=Microsoft CEO takes launch break with the Sun-Times
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| | date=2001-06-01
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| | publisher=Chicago Sun-Times
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| | url=http://suntimes.com/output/tech/cst-fin-micro01.html
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| | archiveurl=http://web.archive.org/web/20010615205548/http://suntimes.com/output/tech/cst-fin-micro01.html
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| | archivedate=2001-06-15
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| }}(Internet archive link)</ref> The term 'viral' may be read as an analogy of [[computer virus]]es. According to [[Free Software Foundation|FSF]] compliance engineer David Turner, it creates a misunderstanding and a fear of using copylefted free software.<ref>{{cite web|url=http://www.linuxtoday.com/developer/2006082902126OSHLLL |title=IT Manager's Journal: 10 Common Misunderstandings About the GPL |first=Bruce |last=Byfield |date=2006-08-29 |accessdate=2008-08-23}}</ref> David McGowan has written that there is no reason to believe the GPL could force proprietary software to become free software, but could "try to enjoin the firm from distributing commercially a program that combined with the GPL’d code to form a derivative work, and to recover damages for infringement." If the firm "actually copied code from a GPL’d program, such a suit would be a perfectly ordinary assertion of copyright, which most private firms would defend if the shoe were on the other foot."<ref>
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| {{Cite book
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| | author = David McGowan
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| | chapter = Legal Aspects of Free and Open Source Software
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| | title = Perspectives on Free and Open Source Software
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| | editor = Joseph Feller, Brian Fitzgerald, Scott A. Hissam, Karim R. Lakahani
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| | publisher = MIT Press
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| | date = 2005
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| | page = 382
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| | isbn = 0-262-06246-1}}</ref>
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| Popular copyleft licenses, such as the GPL, have a clause allowing components to interact with non-copyleft components as long as the communication is abstract, such as executing a command-line tool with a set of switches or interacting with a Web server.<ref>{{cite web|url=http://www.gnu.org/licenses/gpl-faq.html#MereAggregation |title=Frequently Asked Questions about the GNU Licenses |publisher=[[Free Software Foundation]] |date=2008-06-24 |accessdate=2008-08-23}}</ref> As a consequence, even if one module of an otherwise non-copyleft product is placed under the GPL, it may still be legal for other components to communicate with it normally. This allowed communication may or may not include reusing libraries or routines via [[Library (computing)|dynamic linking]] — some commentators say it does,<ref>{{cite web|url=http://www.catb.org/~esr/Licensing-HOWTO.html#id2789302 |title=Licensing HOWTO |date=2002-11-09 |first=Eric Steven |last=Raymond |authorlink=Eric S. Raymond |accessdate=2010-03-21}}</ref> the FSF asserts it does not and explicitly adds an [[GPL linking exception|exception allowing it]] in the license for the [[GNU Classpath]] re-implementation of the Java library.
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| One should also note that on one hand this 'viral' effect is a normal property of any conventional license on derived works of non-copyleft free material, and on the other hand it is the intended effect when using BSD-licensed works as part of [[proprietary software]]. The GNU project using BSD code is in this respect no different from Microsoft or Apple using BSD code, despite claims by proponents of the GPL that it is a freer license than the BSD License.
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| ==Symbol==
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| The copyleft symbol is a backwards C in a circle ([[copyright symbol]] © mirrored). Because it is unavailable on [[Unicode]], it can be approximated with character {{unichar|2184|LATIN SMALL LETTER REVERSED C}} or the more widely available character {{unichar|0254|LATIN SMALL LETTER OPEN O}} between parenthesis '(ɔ)' or, if supported by the application, by combining it with the character {{unichar|20DD|COMBINING ENCLOSING CIRCLE}} 'ↄ⃝'.<ref>{{cite web|url=http://unicode.org/mail-arch/unicode-ml/Archives-Old/UML022/0846.html|title=Unicode copyleft inquiry}}</ref> It has no legal meaning.<ref>{{Cite book|first=G. Brent|last=Hall|year=2008|publisher=Springer|title=Open Source Approaches in Spatial Data Handling|page=29|isbn=3-540-74830-X}} Additional ISBN 978-3-540-74830-4. See {{Google books|JZNuu8XODQMC|Open Source Approaches in Spatial Data Handling}}, [http://books.google.com/books?id=JZNuu8XODQMC&pg=PA29&lpg=PA29&dq=copyleft+symbol+legal+meaning&source=web&ots=bE0wM2RMra&sig=uFpATomM0vQ7yYGNoKMmSERdRpk&hl=en&sa=X&oi=book_result&resnum=6&ct=result page 29].</ref>
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| The Copyleft symbol can be included in HTML using following code (only works for browsers that fully or experimentally support [[CSS3]]'s "transform" property, or Microsoft's "BasicImage" filter):
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| <source lang="html4strict" style="overflow-x:scroll;">
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| <!--[if lte IE 8]><span style="filter: progid:DXImageTransform.Microsoft.BasicImage(rotation=2); display: inline-block;"><![endif]-->
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| <span style="-webkit-transform: rotate(180deg); -moz-transform: rotate(180deg); -o-transform: rotate(180deg); -khtml-transform: rotate(180deg); -ms-transform: rotate(180deg); transform: rotate(180deg); display: inline-block;">
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| ©
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| </span>
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| <!--[if lte IE 8]></span><![endif]-->
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| </source>
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| ==See also==
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| {{Portal|Free software}}
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| {{Div col}}
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| *[[All rights reversed]]
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| *[[Anti-copyright]]
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| *[[Commercial use of copyleft works]]
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| *[[Copyright]]
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| *[[Creative Commons licenses]]
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| *[[CrimethInc. N©! license]] - a license that restricts use by governments and corporations
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| *[[Free Art license]]
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| *[[Free content]]
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| *[[Free Culture movement]]
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| *[[Free Software Foundation]]
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| *[[GNU General Public License]]
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| *[[Hacktivismo Enhanced-Source Software License Agreement|HESSLA]] - a copyleft license which prohibits uses that violate human rights or add spyware
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| *[[Open content]]
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| *[[Open society]]
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| *[[Patentleft]]
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| *[[Permissive free software license]]
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| *[[Public domain]]
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| *[[Share-alike]]
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| *[[WTFPL]] - a "free" license that is absolutely non-copyleft
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| {{Div col end}}
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| ==Notes and references==
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| {{reflist|colwidth=30em}}
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| ==External links==
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| {{Wiktionary|copyleft}}
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| *[http://www.gnu.org/copyleft/copyleft.html What is copyleft?] – by [[Richard Stallman]]
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| *[http://www.gnu.org/bulletins/bull5.html#SEC5 GNU's Bulletin, vol. 1 no. 5] – First appearance of article on ''What is copyleft?''
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| *[http://www.gnu.org/philosophy/freedom-or-power.html Freedom or Power] by Richard Stallman and [[Bradley Kuhn]]
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| *[http://www.gnu.org/philosophy/pragmatic.html Copyleft: Pragmatic Idealism] – by Richard Stallman
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| *[http://www.eyemagazine.com/opinion.php?id=117&oid=290 Eye Magazine – Copyleft and Copyright article]
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| *Two explanations of copyleft and its history by Richard Stallman: one [http://fsfeurope.org/projects/gplv3/fisl-rms-transcript.en.html#what-is-copyleft in April 2006], and one [http://fsfeurope.org/projects/gplv3/barcelona-rms-transcript.en.html#copyleft in June 2006]
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| *[http://www.oreillynet.com/pub/a/policy/2001/12/12/transition.html Article: Working Without Copyleft by Bjørn Reese and Daniel Stenberg, 19 December 2001]
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| * {{Cite book|title= Copy, Rip, Burn: The Politics of Copyleft and Open Source|subtitle=|author=David M. Berry|date=2008|publisher=London:Pluto Press|url=http://www.amazon.com/Copy-Rip-Burn-Politics-Source/dp/0745324142}}
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| * {{Cite book|title= The Success of Open Source|subtitle=|author=Steven Weber|date=2005|publisher= Harvard: Harvard University Press|url=http://www.amazon.com/Success-Open-Source-Steven-Weber/dp/0674018583/ref=ed_oe_p}}
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| {{FOSS}}
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| {{Intellectual property activism}}
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| [[Category:Copyright law]]
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| [[Category:Copyleft| ]]
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| [[Category:Free software]]
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| [[Category:Intellectual property activism]]
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| [[af:Kopielinks]]
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| [[ar:حقوق متروكة]]
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| [[ast:Copyleft]]
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| [[ko:카피레프트]]
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| [[ml:പകർപ്പവകാശത്യജനം]]
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| [[nl:Copyleft]]
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| [[ja:コピーレフト]]
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| [[mhr:Копилефт]]
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| [[ru:Копилефт]]
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| [[simple:Copyleft]]
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| [[ta:அளிப்புரிமை]]
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| [[th:กอปปีเลฟต์]]
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