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A share-alike copyright license clause requires that any improved version of the work be shared on like terms with everyone else—that is, share and share alike.
The GPL and GNU Free Documentation License are share-alike licenses since they require exactly that.
The Creative Commons suite of licenses includes a wider range of share-alike licenses which are denoted usually as "-sa" licenses, e.g.
- CC-by-sa, which requires attribution and is generally considered similar to the GFDL
- CC-nc-sa, which requires non-commercial use only
In the context of Creative Commons only, a share-alike clause states that "If you alter, transform, or build upon this work, you may distribute the resulting work only under a license identical to this one." However, generic variations of share-alike license defines free software and open content. The term copyleft is also used to describe these terms for free software.
Open source, by contrast, does not require share-alike terms to be applied, and permits users to make modifications and improvements and apply a modified and more restrictive license. It is also part of the definition of open source that there can be no restrictions on field of use, which is obviously not the case for all share-alike licenses, e.g. CC-nc-sa.
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