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.

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 reciprocal licenses they have also been described as "viral" due to their self-perpetuating terms.2 Under fair use however the copyleft license may be superseded. 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.3

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 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 licenses such as BSD allow re-distributors to remove some or all these rights and do not require the distribution of source code. History The use of "Copyleft; All Wrongs Reserved" in 1976

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.4 This design did not support text strings and only used integer arithmetic. The goal was for the program to fit in 2 to 3 kilobytes of memory.

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.5 Jim Warren editor of Dr. Dobb's Journal wrote in the July 1976 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 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.5 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".6 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.7 Roger added his name and preserved the COPYLEFT Notice. A later instance of copyleft arose when Richard Stallman was working on a 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.8 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 License9 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. The new license was not at this time given the copyleft label.10 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: "Copyleftall rights reversed."10 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 works. (While the phrase itself appears briefly as (or on) one of his pieces in the 2002 documentary How to Draw a Bunny Johnson himself is not referenced in the 2001 documentary Revolution OS.) Some have suggested that copyleft became a divisive issue in the ideological strife between the Open Source Initiative and the free software movement.11 However there is evidence that copyleft is both accepted and proposed by both parties: Both the OSI and the FSF have copyleft and non-copyleft licenses in their respective lists of accepted licenses.1213 The OSI's original Legal Counsel Lawrence Rosen has written a copyleft license the Open Software License. The OSI's licensing how-to recognises the GPL as a "best practice" license.14 Some of the software programs of Project GNU are published under non-copyleft licenses15 Stallman himself has endorsed the use of non-copyleft licenses in certain circumstances most recently in the case of the Ogg Vorbis license change.16 Applying copyleft 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): 0. the freedom to use the work 1. the freedom to study the work 2. the freedom to copy and share the work with others 3. the freedom to modify the work and the freedom to distribute modified and therefore derivative works. (Note that the list begins from 0 due to a coding tradition the first array element in C and many other programing languages is numbered as 0.) 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 ensure that the author of a derived work can only distribute such works under the same or equivalent license. In addition to restrictions on copying copyleft licenses address other possible impediments. These include ensuring the rights cannot be later revoked and requiring the work and its derivatives to be provided in a form that facilitates modification. In software this requires that the source code of the derived work is made available together with the software itself. 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. 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 the GNU GPL version 2) while in most European countries it is not permitted for a software distributor to 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 EUPL17 or the CeCILL license18 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). Types of copyleft and relation to other licenses See also: Free software licences (Freedom preserving restrictions) 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 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). 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. Strong and weak copyleft 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. "Weak copyleft" licenses are generally used for the creation of 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 glibc (the GNU project's implementation of the C standard library) and then redistributed without any re-licensing required. 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 X11 license Apache license and the BSD licenses. 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 license list but it is not considered compatible with the GPL by the Free Software Foundation. Full and partial copyleft "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). Share-alike 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.19 Some permutations of the Creative Commons licenses are examples of share-alike. Viral licensing Main article: Viral license Copyleft licenses are sometimes referred to as "viral licenses" 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.202122 Many BSD License advocates used the term derisively232425 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."26 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".27 The term 'viral' may be read as an analogy of computer viruses. According to FSF compliance engineer David Turner it creates a misunderstanding and a fear of using copylefted free software.28 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 GPLd code to form a derivative work and to recover damages for infringement." If the firm "actually copied code from a GPLd 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."29 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.30 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 dynamic linking some commentators say it does31 the FSF asserts it does not and explicitly adds an exception allowing it in the license for the GNU Classpath re-implementation of the Java library. 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. Symbol 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 U+2184 latin small letter reversed c between parenthesis () or if supported by the application by combining it with the character U+20DD combining enclosing circle .32 It has no legal meaning.33 See also Free software portal All rights reversed Anti-copyright Commercial use of copyleft works Copyright Creative Commons licenses CrimethInc. N! license - a license that restricts use by governments and corporations Free Art license Free content Free Culture movement Free Software Foundation GNU General Public License HESSLA - a copyleft license which prohibits uses that violate human rights or add spyware Open content Open society Patentleft Permissive free software licence Public domain Share-alike WTFPL - a "free" licence that is absolutely non-copyleft Notes and references "What is Copyleft". http://www.gnu.org/copyleft. Retrieved 2010-08-29.  Mundie Craig (May 3 2001). "Prepared Text of Remarks by Craig Mundie Microsoft Senior Vice President - The Commercial Software Model". New York University Stern School of Business. http://www.microsoft.com/presspass/exec/craig/05-03sharedsource.mspx. Retrieved 2009-10-01.  Brian Still & Kirk St. Amant (2007). "Examining Open Source Software Licenses through the Creative Commons Licensing Model". Handbook of Research on Open Source Software: Technological Economic and Social Perspectives. Information Science Reference. pp. 382 of 728. ISBN 1591409993.  Allison Dennis (July 1976). "Design notes for TINY BASIC". SIGPLAN Notices (ACM) 11 (7): pp. 2533. doi:10.1145/987491.987494.  The ACM Special Interest Group on Programming Languages (SIGPLAN) reprinted the Tiny Basic design notes from the January 1976 Tiny BASIC Journal. a b Warren Jim C. (July 1976). "Correspondence". SIGPLAN Notices (ACM) 11 (7): pp. 12. ISSN 0362-1340.  Wang Li-Chen (May 1976). "Palo Alto Tiny BASIC". Dr. Dobb's Journal of Computer Calisthenics & Orthodontia Running Light Without Overbyte 1 (5): 1225.  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. Rauskolb Roger (December 1976). "Dr. Wang's Palo Alto Tiny BASIC". Interface Age 2 (1): 92108.  The source code begins with the following nine lines: 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 Williams Sam (March 2002). "7". Free as in Freedom - Richard Stallman's Crusade for Free Software. O'Reilly Media. ISBN 0-596-00287-4. http://www.oreilly.com/openbook/freedom/ch07.html.  "Emacs General Public License". 2001-07-05. http://www.free-soft.org/gplhistory/emacsgpl.html. Retrieved 2008-08-23.  a b Stallman Richard (2008-01-21). "About the GNU Project". Free Software Foundation. http://www.gnu.org/gnu/thegnuproject.html. Retrieved 2008-08-23.  Biancuzzi Federico (2005-06-30). "ESR: "We Don't Need the GPL Anymore"". ONLamp.com. http://www.onlamp.com/pub/a/onlamp/2005/06/30/esrinterview.html. Retrieved 2008-08-23.  Tiemann Michael (2006-09-18). "Licenses by Name". http://www.opensource.org/licenses/alphabetical. Retrieved 2008-08-23.  "Licenses". 2008-06-05. http://www.fsf.org/licensing/licenses. Retrieved 2008-08-23.  Raymond Eric Steven (2002-11-09). "Licensing HOWTO". http://www.catb.org/esr/Licensing-HOWTO.html#choosing. Retrieved 2008-08-23.  What the GPLv3 Means for MS-Novell Agreement Stallman Richard (2001-02-26). "LWN.net: RMS on the Ogg Vorbis license". http://lwn.net/2001/0301/a/rms-ov-license.php3. Retrieved 2008-08-23. "My 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."  "The EUPL - European Union Public Licence". European Commission. http://www.osor.eu/eupl/european-union-public-licence-eupl-v.1.1. Retrieved 2007-01-09.  "Free Software Licensing Agreement CeCILL". INRIA. http://www.inria.fr/valorisation/logiciels/Licence.CeCILL-V1.US.pdf. Retrieved 2010-08-24.  Rob Myers NonCommercial Sharealike is not Copyleft Vixie Paul (2006-03-06). "Re: Section 5.2 (IPR encumberance) in TAK rollover requirement draft". IETF Namedroppers mailing list. Archived from the original on 2007-09-27. http://web.archive.org/web/20070927175628/http://psg.com/lists/namedroppers/namedroppers.2006/msg00246.html. Retrieved 2007-04-29.  "General Public Virus". Jargon File 2.2.1. 1990-12-15. http://catb.org/esr/jargon/oldversions/jarg221.txt. Retrieved 2007-04-29.  Hackvn Stig (September 1999). "Reverse-engineering the GNU Public Virus Is copyleft too much of a good thing". Linux Journal. http://devlinux.org/lw-gnu-published.html. Retrieved 2007-04-29.  Stewart Bill (1998-10-08). "Re: propose: cypherpunks license' (Re: Wanted: Twofish source code)". Cypherpunks mailing list. http://cypherpunks.venona.com/date/1998/10/msg00429.html. Retrieved 2007-04-29.  Buck Joe (2000-10-10). "Re: Using of parse tree externally". GCC mailing list. http://gcc.gnu.org/ml/gcc/2000-10/msg00198.html. Retrieved 2007-04-29.  Griffis L. Adrian (2000-07-15). "The GNU Public Virus". http://themes.freshmeat.net/articles/view/172#comment-5548. Retrieved 2007-04-29.  Mundie Craig (2001-05-03). "Speech Transcript - Craig Mundie". New York University Stern School of Business. http://www.microsoft.com/presspass/exec/craig/05-03sharedsource.mspx. Retrieved 2008-08-23.  Newbart Dave (2001-06-01). "Microsoft CEO takes launch break with the Sun-Times". Chicago Sun-Times. Archived from the original on 2001-06-15. http://web.archive.org/web/20010615205548/http://suntimes.com/output/tech/cst-fin-micro01.html. (Internet archive link) Byfield Bruce (2006-08-29). "IT Manager's Journal: 10 Common Misunderstandings About the GPL". http://www.linuxtoday.com/developer/2006082902126OSHLLL. Retrieved 2008-08-23.  David McGowan (2005). "Legal Aspects of Free and Open Source Software". In Joseph Feller Brian Fitzgerald Scott A. Hissam Karim R. Lakahani. Perspectives on Free and Open Source Software. MIT Press. p. 382. ISBN 0262062461.  "Frequently Asked Questions about the GNU Licenses". Free Software Foundation. 2008-06-24. http://www.gnu.org/licenses/gpl-faq.html#MereAggregation. Retrieved 2008-08-23.  Raymond Eric Steven (2002-11-09). "Licensing HOWTO". http://www.catb.org/esr/Licensing-HOWTO.html#id2789302. Retrieved 2010-03-21.  "Unicode copyleft inquiry". http://unicode.org/mail-arch/unicode-ml/Archives-Old/UML022/0846.html.  Hall G. Brent (2008). Open Source Approaches in Spatial Data Handling. Springer. p. 29. ISBN 354074830X.  Additional ISBN 9783540748304. See Open Source Approaches in Spatial Data Handling at Google Books page 29. External links Look up copyleft in Wiktionary the free dictionary. What is copyleft by Richard Stallman GNU's Bulletin vol. 1 no. 4 First appearance of article on What is copyleft Freedom or Power by Richard Stallman and Bradley Kuhn Copyleft: Pragmatic Idealism by Richard Stallman Eye Magazine Copyleft and Copyright article Two explanations of copyleft and its history by Richard Stallman: one in April 2006 and one in June 2006 Article: Working Without Copyleft by Bjrn Reese and Daniel Stenberg 19 December 2001 David M. Berry (2008). Copy Rip Burn: The Politics of Copyleft and Open Source. London:Pluto Press. http://www.amazon.com/Copy-Rip-Burn-Politics-Source/dp/0745324142.  Steven Weber (2005). The Success of Open Source. Harvard: Harvard University Press. http://www.amazon.com/Success-Open-Source-Steven-Weber/dp/0674018583/refedoep.  v d eFree and open source software General Copyleft  Events and Awards  Free software  Free Software Definition  Gratis versus libre  List of free and open source software packages  List of project directories/Comparison of hosting facilities  Open-source software Operating system families AROS  BSD  Darwin  eCos  FreeDOS  GNU  Haiku  Inferno  Linux  Mach  MINIX  OpenSolaris  Plan 9  ReactOS  Symbian Development Eclipse  Free Pascal  GCC  Gambas  Java  LLVM  Lua  NetBeans  Open64  Perl  PHP  Python  ROSE  Ruby  Tcl History GNU  Haiku  Linux  Mozilla (Application Suite  Firefox  Thunderbird) Organizations Apache Software Foundation  Blender Foundation  Eclipse Foundation  FreeBSD Foundation  freedesktop.org  Free Software Foundation (Europe  India  Latin America)  FSMI  GNOME Foundation  GNU Project  Google Code  KDE e.V.  Linux Foundation  Mozilla Foundation  Open Source Geospatial Foundation  Open Source Initiative  Software Freedom Conservancy  SourceForge  Symbian Foundation  The Document Foundation  Xiph.Org Foundation  XMPP Standards Foundation  X.Org Foundation Licences Apache  Artistic  BSD  GNU GPL  GNU LGPL  ISC  MIT  MPL  Ms-PL/RL  zlib  Public domain/CC0  FSF approved licenses Licence standards Open Source Definition  The Free Software Definition  Debian Free Software Guidelines Challenges Binary blob  Canonical's contributor agreement  Digital rights management  Graphics hardware compatibility  License proliferation  Mozilla software rebranding  Proprietary software  SCO-Linux controversies  Security  Software patents  Hardware restrictions  Trusted Computing  Viral license Other topics Alternative terms  Community  Linux distribution  Forking  Movement  Microsoft Open Specification Promise  Revolution OS  Comparison with closed source Book:Free and Open Source Software   Category:Free software   Portal:Free software v d eIntellectual property activism Issues and debates Copyright infringement  Criticism of intellectual property  Criticism of patents  Digital rights management  Gripe site  History of music piracy  Mashup videos and music  Public domain  Software patent debate Concepts All rights reversed  Copyleft  Commercial use of copyleft works  Commons-based peer production  Free content  Free software licence  Infoanarchism  Libertarian positions  Open content  Open design  Open Music Model  Open patent  Open source hardware  Open source software  Share-alike Movements Access to Knowledge movement  Anti-copyright  Cultural environmentalism  Free culture movement  Free software movement Organizations Creative Commons  Electronic Frontier Foundation  Free Software Foundation  Open Rights Group  The Pirate Bay  Piratbyrn  Pirate Party  Students for Free Culture Documentaries Steal This Film (2006 2007)  Good Copy Bad Copy (2007)  RiP!: A Remix Manifesto (2008)