Copyleft is a play on the word copyright and is the practice of using copyright law to remove restrictions on distributing copies and modified versions of a work for others and requiring that the same freedoms be preserved in modified versions.
Copyleft is a form of licensing and may be used to modify copyrights for works such as computer software, documents, music, and art. In general, copyright law allows an author to prohibit others from reproducing, adapting, or distributing copies of the author's work. In contrast, an author may, through a copyleft licensing scheme, give every person who receives a copy of a work permission to reproduce, adapt or distribute the work as long as any resulting copies or adaptations are also bound by the same copyleft licensing scheme. A widely used and originating copyleft license is the GNU General Public License. Similar licenses are available through Creative Commons - called Share-alike.
Copyleft may also 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 copyright restrictions are imposed), copyleft allows an author to impose some but not all copyright restrictions on those who want to engage in activities that would otherwise be considered copyright infringement. Under copyleft, copyright infringement may be avoided if the would-be infringer perpetuates the same copyleft scheme. For this reason copyleft licenses are also known as reciprocal licenses.
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. This design did not support text strings and only used integer arithmetic. The goal was for the program to fit in 2 to 3 kbytes 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 very successful open source project.
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". 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. 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.
As Stallman deemed it impractical in the short term to eliminate current copyright law and the wrongs he perceived it perpetuating, he decided to work within the framework of existing law; he created his own copyright license, the Emacs General Public License , 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.
Richard Stallman stated that it comes from Don Hopkins, whom he calls a very imaginative fellow, who mailed him a letter in 1984 or 1985 on which was written: "Copyleft—all rights reversed."  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.)
There are definitional problems with the term "copyleft" which contribute to controversy over it. The term originated as an amusing back-formation from the term "copyright", and was originally a noun, meaning the copyright license terms of the GNU General Public License originated by Richard Stallman as part of the Free Software Foundation's work. Thus, "your program is covered by the copyleft" is almost considered to mean the same as the program being GPL'd. When used as a verb, as in "he copylefted his most recent version", it is less precise and can refer to any of several similar licenses, or indeed a notional imaginary license for discussion purposes.
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):
- the freedom to use and study the work,
- the freedom to copy and share the work with others,
- the freedom to modify the work,
- and the freedom to distribute modified and therefore derivative works.
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 are 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 CeCILL license, a license that allows one to use GNU GPL (see article 5.3.4 of CeCILL) in combination with a limited warranty (see article 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. Copyleft even became a divisive issue in the ideological strife between the Open Source Initiative and the free software movement. 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. The result is however that it can be used for any kind of document.
Strong and weak copyleft
The copyleft governing a work is considered to be "stronger", to 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, documentation, or art, such as 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 is when all parts of a work (except the license itself) can be modified by consecutive authors. Partial copyleft exempts some parts of the work from the copyleft provisions, thus permitting unrestricted modification, or in some other way does not impose all the principles of copylefting on the work. For example, in artistic creation full copylefting is sometimes not possible or desirable (see below).
Many share-alike licenses are partial (or non-full) copyleft licenses. Share-alike, however, imposes the requirement that any freedom that is granted regarding the original work (or its copies), must be granted on exactly the same terms in any derived work: this further implies that any full copyleft license is automatically a share-alike license (but not the other way around!). Instead of using copyright's "all rights reserved" motto, or full copyleft's "no rights reserved", share-alike licenses rather use the "some rights reserved" statement. Some permutations of the Creative Commons licenses are examples of share-alike.
Copyleft licenses are sometimes referred to as viral copyright licenses, because any works derived from a copyleft work must themselves be copyleft when distributed. The term GPV or "GNU General Public Virus" has a long and rich history on the Internet, dating back to shortly after the GPL was first conceived. Many 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. More recently, Microsoft has used language with this term. The term viral is used 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.
The terms of traditional copyright are more restrictive than any imposed by copyleft licenses. 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 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.
In the sense that redistributions and copies of copyleft material will always remain copyleft, the applicability of the adjective viral is trivial. It is also trivially applicable, ironically enough, to copyright. In the case of copyright, however, the viral agent has more restricted fecundity. In memetic terms, copyleft has higher fitness (if this is taken to be analogous to the meaning of the term in genetics).
- All Rites Reversed or Kopyleft
- Commercial use of copyleft works
- Copyleft art
- Copyleft patent
- Creative Commons licenses
- Free culture movement
- Free Software Foundation
- Glossary of legal terms in technology
- Public domain
- GNU General Public License
- Permissive and copyleft licences
Notes and references
- ^ Wang, Li-Chen (May 1976). "Palo Alto Tiny BASIC". Dr. Dobb's Journal of Computer Calisthenics & Orthodontia, Running Light Without Overbyte. 1 (5): 12-25. 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): 92-108. 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
- ^ Vixie, Paul (2006-03-06). Re: Section 5.2 (IPR encumberance) in TAK rollover requirement draft. IETF Namedroppers mailing list. Retrieved on 2007-04-29.
- ^ General Public Virus. Jargon File 2.2.1 (1990-12-15). Retrieved on 2007-04-29.
- ^ Hackvän, Stig (September 1999). "Reverse-engineering the GNU Public Virus — Is copyleft too much of a good thing?". Linux Journal. Retrieved on 2007-04-29.
- ^ Stewart, Bill (1998-10-08). Re: propose: `cypherpunks license' (Re: Wanted: Twofish source code). Cypherpunks mailing list. Retrieved on 2007-04-29.
- ^ Buck, Joe (2000-10-10). Re: Using of parse tree externally. GCC mailing list. Retrieved on 2007-04-29.
- ^ Griffis, L. Adrian (2000-07-15). The GNU Public Virus. Retrieved on 2007-04-29.
- ^ http://www.microsoft.com/presspass/exec/craig/05-03sharedsource.mspx
- ^ Brucy Byfield. 10 common misunderstandings about the GPL.
- 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
- Linus Torvalds on commercial potential of Linux software (October 2004 interview)
- 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