Copyleft

Intellectual property
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Copyleft is the legal technique of granting certain freedoms over copies of copyrighted works with the requirement that the same rights be preserved in derivative works. In this sense, freedoms refers to the use of the work for any purpose, and the ability to modify, copy, share, and redistribute the work, with or without a fee. Licenses which implement copyleft can be used to maintain copyright conditions for works ranging from computer software, to documents, art, and scientific discoveries, and similar approaches have even been applied to certain patents.^[1]

Copyleft software licenses are considered protective or reciprocal in contrast with permissive free software licenses,^[2] and require that information necessary for reproducing and modifying the work must be made available to recipients of the software program, which are often distributed as executables. This information is most commonly in the form of source code files, which usually contain a copy of the license terms and acknowledge the authors of the code. Copyleft helps ensure everyone's rights to freely use the product but it prohibits owning, registering copyright and earning royalties from copyright.

Notable copyleft licenses include the GNU General Public License (GPL), originally written by Richard Stallman, which was the first software copyleft license to see extensive use;^{[3][non-primary source needed]} the Mozilla Public License; the Free Art License;^{[4][non-primary source needed]} and the Creative Commons share-alike license condition^{[5][non-primary source needed]}—with the last two being intended for non-software works, such as documents and pictures, both academic or artistic in nature. Wikipedia is copyleft under the Creative Commons Attribution-ShareAlike license.