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Copyleft is the application of copyright law to ensure public freedom to manipulate, improve, and redistribute a work of authorship and all derivative works.

In copyleft, the copyright holder grants an irrevocable license to the recipient of a copy, permitting the redistribution (including sale) of possibly modified further copies, under the condition that all those copies carry the same license and are made available in a form which facilitates modification.

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. A longstanding issue of debate is 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, while non-copyleft free software licenses maximize the freedom of the initial recipient.

The copyleft concept was invented by Richard Stallman in 1984 for the distribution of computer software, although it is now also being used for other types of material. The term "copyleft" came from a message contained in Tiny BASIC, a free distributed version of Basic written by Dr. Wang in the late 70's. The program listing contained the following phrases "All Wrongs reserved" and "CopyLeft",

Free software licenses that use copyleft include the GNU General Public License, the GNU Lesser General Public License, the Mozilla Public License, and the Q Public License.

Copyleft licenses for materials other than software include the Open Content License, and the GNU Free Documentation License. The latter is being used for the content of Wikipedia. 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 copylefted works to harmful computer viruses. However, the analogy between copyleft and computer viruses is not close; as advocates of copyleft point out, computer viruses infect computers without the awareness of the user, whereas the copyleft actually grants the user certain permissions to distribute modified programs, which is otherwise not allowed under copyright law or by most proprietary licenses. Furthermore, copyright itself would be "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 mainly supported by Microsoft. They say that if your product uses GPLed code, your product automatically escapes your control, becomes GPLed right away and you can't do anything about it. This is not true since the GPL (and all other copyleft licenses) are licenses and not contracts. To make it clear, let's compare it to a fishing license :

- You accept the license by buying it, since you can't legally fish otherwise. - You accept the copyleft license by distributing the software since you can't distribute it otherwise. - If you break the terms of the fishing license (fishing more than your quota, for example), you can lose it and be fined. The only thing that can be claimed from you is money. - If you break the term of the copyleft license, you can be fined, pay damages to the copyright holder of the misused code and attorney fees. Then, you must make a choice : either remove the code from your software and write your own (since you still have no right to use it) or make your software copyleft (and have the right to use it).

The concept of copyleft arose when 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 proceeded to create a software license that would prevent this behavior.

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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