Copyleft distinguished from copyright, is the practice of offering people the right to freely distribute copies and derivative work (modified versions) of a original work with the stipulation that the same rights be preserved in derivative works created later.

Copyleft software licenses are considered protective or reciprocal, as contrasted with permissive Free Software licenses

Copyleft type licenses are a novel use of existing copyright law to ensure a Original work remains freely available. The GNU General Public License (GPL), originally written by Richard Stallman, was the first software Copyleft license to see extensive use, and continues to dominate in that area.

Creative Commons, a Not-for-profit Entity founded by Lawrence Lessig, provides a similar license-provision condition called share-alike

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