Copyleft

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. Similar approaches have been applied to certain patents. Copyleft software licenses are considered protective or reciprocal (in contrast with permissive free software licenses): they require that information necessary for reproducing and modifying the work be made available to recipients of the software program. 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 licenses are designed to ensure that the same rights granted to users by the work's authors are preserved when that work (or modified versions of it) are redistributed. 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; the Mozilla Public License; the Free Art License; and the Creative Commons ShareAlike license condition – with the last two 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.