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Copyleft is the application of copyright law to force derivative works to also be released with a copyleft license. So long as all of those wanting to modify the work accept the terms, the net effect is to facilitate successive improvement by a wide range of contributors. Those who are unwilling or unable to accept the terms are prohibited from creating derivative works.

No restrictions apply to works in the public domain. They may be freely modified, and the creator of the derivative work may license any new portions of the derivative work, but not the public domain portion, under any terms, or none. The resulting derivative work may not be available to the creators of the original or may compete with them.

In copyleft, the copyright holder grants an irrevocable license to the recipient of a copy, generally permitting the free unlimited use, modification and redistribution (often including sale of media or auxiliary materials which may carry a different copyright license (e.g. documentation)) of copies. The distinctive condition to that license is that any modifications to the work, if redistributed, must carry the same permissions (i.e. license terms) and be made available in a form which facilitates modification. For software, this means in source code.

The concept 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 create a software license that would prevent this behavior which he named software hoarding. The term "copyleft" came from a message contained in Tiny BASIC, a free distributed version of Basic written by Dr. Wang in the late 1970s. The program listing contained the phrases "All Wrongs reserved" and "CopyLeft."

There are definitional problems with the term "copyleft" which contribute to controversy over it. The term originated as an amusing backformation 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'. When used as a verb (i.e. 'he copylefted his most recent version'), it is less precise and can refer to any of several similar licenses, or indeed to a notional imaginary license for discussion purposes. It is the last case which generates more heat than light, as each discussant may be referring to his own notional license.

Because of complications caused by use of software library routines, there developed the GNU Library General Public License (subsequently renamed the Lesser GPL), which changes the requirement of further distribution in ways which are compatible with actual library routine use.

Copyleft is one of the key features in free software/open source licences, and is the licenses' legal framework to ensure that derivatives of the licensed work stay free/open. If the licensee fails to distribute derivative works under the same license he will face legal consequences - the license is terminated, leaving the licensee without permission to copy, distribute, display publicly, or prepare derivative works of the software.

Other free software licenses, such as those used by the BSD operating systems, the X Window System and the Apache web server, 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 a larger degree of freedom. This debate hinges on complex issues such as the definition of freedom and whose freedoms are more important. It is sometimes argued that the copyleft licenses attempt to maximize the freedom of all potential recipients in the future (freedom from the creation of proprietary software), while non-copyleft free software licenses maximize the freedom of the initial recipient (freedom to create proprietary software).

An example of a Free software licenses 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 Q Public License, the X11 license, and the BSD licenses.

Copyleft licenses for materials other than software include the Creative Commons ShareAlike licenses and the GNU Free Documentation License. The latter is being used for the content of Wikimd. The Free Art license is a license that can be applied to any work of art.

Copyleft licenses are sometimes called viral copyright licenses because any works derived from a copylefted work must themselves be copylefted. This term is considered derogatory, as it compares copyleft licenses to viruses (computer or otherwise). Most advocates of copyleft argue that the analogy between copyleft and computer viruses does not apply. As they point out, computer viruses generally infect computers without the awareness of the user, whereas the copyleft actually grants the user certain permissions to distribute modified programs, which is not allowed under copyright law without permission of the copyright holder. Most proprietary software licenses do not allow such distribution. Furthermore, copyright itself is "viral" in this sense, since any works derived from a copyrighted work must have permission from and obey any conditions set by the original copyright holder.

The view that copyleft licenses are viral is supported by Microsoft, who say that if a product uses GPLed code, that product automatically escapes the creator's control and becomes GPLed, leaving the creator no recourse. Advocates, including Eben Moglen, Professor of Law at Columbia University and counsel for the Free Software Foundation, note that this is not true since the GPL is a license, not a contract.

Microsoft, and others, in describing the GPL as a "viral license", may also be referring to the idea that any release of something new under the GPL would seem to create a positive feedback network effect, in which over time there will be an ever-expanding amount of copylefted code. Code reuse is of course tempting, as a way to save effort and get on with a project, especially when a perfectly sensible design and implementation has already been done and is available. In contrast, those working on non-copylefted programs will have to "reinvent the wheel" for their own programs.

It is sometimes suggested that software under copyleft is resistant to unnecessary forking because all maintainers, of the original work or other versions, may use any modifications released by anyone. Useful changes tend to be merged, and different versions are maintained only to the extent that they are useful. It is thought that Linux has not suffered the same fragmentation as Unix because it is copylefted.

Copyleft-like ideas are increasingly being suggested for patents, such as open patent pools that allow royalty-free use of patents contributed to the pool under certain conditions (such as surrendering the right to apply for new patents that are not contributed to the pool).

Copyleft is also starting to inspire the arts with movements like the Libre Society and open-source record labels emerging.

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