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Copyleft (a play on the word copyright) is the practice of offering people the right to freely distribute copies and modified versions of a work with the stipulation that the same rights be preserved in derivative works down the line. Copyleft software licenses are considered protective or reciprocal, as contrasted with permissive free software licenses.

Copyleft is a form of licensing, and can be used to maintain copyright conditions for works ranging from computer software, to documents, to art, to scientific discoveries and instruments in medicine. In general, copyright law is used by an author to prohibit recipients from reproducing, adapting, or distributing copies of their work. In contrast, under copyleft, an author must give every person who receives a copy of the work permission to reproduce, adapt, or distribute it, with the accompanying requirement that any resulting copies or adaptations are also bound by the same licensing agreement.

Copyleft licenses for software require that information necessary for reproducing and modifying the work must be made available to recipients of the binaries. The source code files will usually contain a copy of the license terms and acknowledge the authors.

Copyleft type licenses are a novel use of existing copyright law to ensure a work remains freely available. The GNU General Public License (GPL), originally written by Richard Stallman, was the first software copyleft license to see extensive use, and continues to dominate in that area. Creative Commons, a non-profit organization founded by Lawrence Lessig, provides a similar license provision condition called share-alike.

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):

Freedom 0 – the freedom to use the work,

Freedom 1 – the freedom to study the work,

Freedom 2 – the freedom to copy and share the work with others,

Freedom 3 – the freedom to modify the work, and the freedom to distribute modified and therefore derivative works.

(Note that the list begins from 0 as a reference to computer programming, where zero-based numbering is prevalent.)

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 be 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 EUPL, or the CeCILL license, 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).For projects which will be run over a network, a variation of the GPL is provided in the Affero General Public License, which ensures that the source code is available to users of network software.

Types and relation to other licenses

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. A stronger copyleft license is the AGPL which requires the publishing of the source code also for the usage case software as a service, ("see also" the term sometimes used, "service as a software substitute ") e.g. when software is deployed on servers. The Sybase Open Watcom Public License is among the strongest copyleft license, as this license closes the "private usage" loophole of the GPL, and requires the publishing of source code in any use case.

Free software licenses that use "weak" copyleft include the GNU Lesser General Public License and the Mozilla Public License. Examples of non-copyleft ("permissive") free software licenses include the X11 license, Apache license and the BSD licenses. The concrete effect of strong vs. weak copyleft has yet to be tested in court.

The Design Science License is a strong copyleft license that can apply to any work -- not only software or documentation, but also to literature, artworks, music, photography, and video. It was created by Michael Stutz after his interest in applying GNU-style copyleft to non-software works, which later came to be called Open content. In the 1990s the DSL was used on music recordings, visual art, and even novels. It is now 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, by contrast, 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. 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. Some permutations of the Creative Commons licenses are examples of share-alike.

Viral licensing

Viral license is a pejorative name for copyleft licenses. It originates from the terms 'General Public Virus' or 'GNU Public Virus' (GPV), which dates back to 1990, a year after the GPLv1 was released. The name "viral licenses" refers to the fact that any works derived from a copyleft work must preserve the copyleft permissions when distributed.

Some BSD License advocates used the term derisively 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." 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 is licensed under a GPL compatible license, and described it thus as "a cancer that attaches itself in an intellectual property sense to everything it touches".

In response to Microsoft's attacks on the GPL, several prominent Free Software developers and advocates released a joint statement supporting the license. According to FSF compliance engineer David Turner, it creates a misunderstanding and a fear of using copylefted free software. 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."

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. 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 does, 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. This ambiguity is an important difference between the GPL and the LGPL, in that the LGPL specifically allows linking or compiling with the covered work.

Symbol

The copyleft symbol is a backwards C in a circle (copyright symbol © mirrored). It has no legal significance. A 2016 proposal to add the symbol to a future version of Unicode was accepted by the Unicode Technical Committee. The code point U+1F12F 🄯 COPYLEFT SYMBOL was added in Unicode 11.

As of 2018, it is currently largely unimplemented in fonts, but can be approximated with character U+2184 ↄ LATIN SMALL LETTER REVERSED C or the more widely available character U+0254 ɔ LATIN SMALL LETTER OPEN O between parenthesis '(ɔ)' or, if supported by the application, by combining it with the character U+20DD ↄ⃝ COMBINING ENCLOSING CIRCLE: 'ↄ⃝'.

See also

• All rights reversed
• Opposition to copyright
• Comparison of open source and closed source
• Commercial use of copyleft works
• Copyright
• Creative Commons licences
• Free content
• Free Culture movement
• Free Software Movement
• GNU General Public License
• HESSLA – a license which prohibits uses that violate human rights or add spyware
• History of free and open-source software
• Open content
• Patentleft
• Permissive free software licence
• Public copyright license
• Public domain
• Share-alike

Notes and references

1. "What is Copyleft?". Retrieved 29 August 2010.
2. Open Source from a Proprietary Perspective at the Wayback Machine (archive index)
3. Newman, John (29 December 2011). "Copyright and Open Access at the Bedside". NEJM. 365: 2447–2449. doi:10.1056/NEJMp1110652. Retrieved 25 May 2017.
4. Stallman, Richard (29 June 2007). "GNU General Public License". GNU. Free Software Foundation. Retrieved 14 August 2015.
5. "Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)". Creative Commons. Creative Commons. Retrieved 14 August 2015.
6. "The EUPL – European Union Public Licence". European Commission. Retrieved 9 January 2007.
7. "Free Software Licensing Agreement CeCILL" (PDF). INRIA. Retrieved 24 August 2010.
8. For some of the philosophy behind the term "SaaSS", see the "Server Software" section of "How to choose a license for your own work". The GNU Project. 20 December 2015. Retrieved 4 July 2017.
9. Stallman, Richard (18 November 2016). "Who does that server really serve?". The GNU Project. Retrieved 4 July 2017.
10. "Why the Affero GPL". The GNU Project. 10 May 2015. Retrieved 4 July 2017.
11. List of free-software licences on the FSF website: “We recommend that developers consider using the GNU AGPL for any software which will commonly be run over a network”.
12. "Various Licenses and Comments about Them - Sybase Open Watcom Public License version 1.0 (#Watcom)". gnu.org. Retrieved 23 December 2015. This is not a free software license. It requires you to publish the source code publicly whenever you "Deploy" the covered software, and "Deploy" is defined to include many kinds of private use.
13. Patrice-Emmanuel Schmitz (3 April 2015). "The meaning of "Copyleft" in EUPL". Joinup.
14. Various Licenses and Comments about Them
15. Engelfriet, Arnoud. "Choosing an Open Source License". Athens University of Economics and Business. Archived from the original on 15 July 2012. Retrieved 6 February 2012. {{cite web}}: Unknown parameter |dead-url= ignored (|url-status= suggested) (help)
16. Rob Myers, NonCommercial Sharealike is not Copyleft
17. "Microsoft license spurns open source". CNET. CBS Interactive.
18. "Some rights reserved: the alternatives to copyright". Wired UK.
19. "Glossary". a2knetwork.org. Archived from the original on 26 April 2009. {{cite web}}: Unknown parameter |dead-url= ignored (|url-status= suggested) (help)
20. Jason Greenberg; Daniel Glazer (March 2013). "Inoculating Your Purchase – Contractual Protection from Viral Licenses in M&A Transactions" (PDF). Fried Frank. Association of Corporate Counsel. Retrieved 10 June 2016.
21. Philip Wood; Amy Ryburn; Allan Yeoman; Andrew Matangi; Steve Nightingale (1 July 2013). "Legal update on Information and Communication Technology – July 2013". Archived from the original on 21 September 2013. Retrieved 10 June 2016. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
22. Vixie, Paul (6 March 2006). "Re: Section 5.2 (IPR encumberance) in TAK rollover requirement draft". IETF Namedroppers mailing list. Archived from the original on 27 September 2007. Retrieved 29 April 2007.
23. "General Public Virus". Jargon File 2.2.1. 15 December 1990. Retrieved 29 April 2007.
24. Hackvän, Stig (September 1999). "Reverse-engineering the GNU Public Virus – Is copyleft too much of a good thing?". Linux Journal. Archived from the original on 18 July 2011. Retrieved 29 April 2007. {{cite journal}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
25. Stewart, Bill (8 October 1998). "Re: propose: 'cypherpunks license' (Re: Wanted: Twofish source code)". Cypherpunks mailing list. Archived from the original on 29 May 2007. Retrieved 29 April 2007. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
26. Buck, Joe (10 October 2000). "Re: Using of parse tree externally". GCC mailing list. Retrieved 29 April 2007.
27. Griffis, L. Adrian (15 July 2000). "The GNU Public Virus". Retrieved 29 April 2007.
28. Mundie, Craig (3 May 2001). "Speech Transcript – Craig Mundie". New York University Stern School of Business. Archived from the original on 21 June 2005. Retrieved 23 August 2008. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
29. Newbart, Dave (1 June 2001). "Microsoft CEO takes launch break with the Sun-Times". Chicago Sun-Times. Archived from the original on 15 June 2001.(Internet archive link)
30. Free Software Leaders Stand Together  – via Wikisource.
31. Byfield, Bruce (29 August 2006). "IT Manager's Journal: 10 Common Misunderstandings About the GPL". Retrieved 23 August 2008.
32. David McGowan (2005). "Legal Aspects of Free and Open Source Software". In Joseph Feller; Brian Fitzgerald; Scott A. Hissam; Karim R. Lakahani (eds.). Perspectives on Free and Open Source Software. MIT Press. p. 382. ISBN 0-262-06246-1.
33. "Frequently Asked Questions about the GNU Licenses". Free Software Foundation. 24 June 2008. Retrieved 23 August 2008.
34. Raymond, Eric Steven (9 November 2002). "Licensing HOWTO". Retrieved 21 March 2010.
35. Stallman, Richard. Why you shouldn't use the Lesser GPL for your next library. Free Software Foundation official website.
36. Hall, G. Brent (2008). Open Source Approaches in Spatial Data Handling. Springer. p. 29. ISBN 9783540748311.
37. "Proposal to add the Copyleft Symbol to Unicode" (PDF).
38. ^ "Proposed New Characters: Pipeline Table". Unicode Character Proposals. Unicode Consortium. Retrieved 18 April 2017.
39. "Announcing The Unicode® Standard, Version 11.0". blog.unicode.org. Retrieved 6 June 2018.
40. "Unicode copyleft inquiry". 6 May 2000.

External links

• What is copyleft? – by Richard Stallman.
• GNU's Bulletin, vol. 1 no. 5 – first appearance of article on What is copyleft?
• Freedom or Power by Richard Stallman and Bradley Kuhn.
• Copyleft: Pragmatic Idealism – by Richard Stallman.
• "Copyleft and Copyright article". Eye Magazine.
• Two explanations of copyleft and its history by Richard Stallman: one in April 2006, and one in June 2006.
• Article: Working Without Copyleft by Bjørn Reese and Daniel Stenberg, 19 December 2001.
• David M. Berry (2008). Copy, Rip, Burn: The Politics of Copyleft and Open Source. London: Pluto Press. {{cite book}}: Cite has empty unknown parameter: |subtitle= (help)
• Steven Weber (2005). The Success of Open Source. Harvard: Harvard University Press. {{cite book}}: Cite has empty unknown parameter: |subtitle= (help)

• Copyright law
• Copyleft
• Free software
• Intellectual property activism
• Free and open-source software licenses