Share-alike is a copyright licensing term, originally used by theCreative Commons project, to describe works or licenses that require copies or adaptations of the work to be released under the same or similar license as the original.[1]Copyleft licenses arefree content orfree software licenses with a share-alike condition.
Two currently-supported Creative Commons licenses have the ShareAlike condition: Creative Commons Attribution-ShareAlike (acopyleft,free content license) and Creative Commons Attribution-NonCommercial-ShareAlike (a proprietary license).
The term has also been used outside copyright law to refer to a similar plan forpatent licensing.[2]
Copyleft orlibre share-alike licenses are the largest subcategory of share-alike licenses. They include bothfree content licenses like Creative Commons Attribution-ShareAlike andfree software licenses like theGNU General Public License. These licenses have been described pejoratively asviral licenses, because the inclusion of copyleft material in a larger work typically requires the entire work to be made copyleft. The termreciprocal license has also been used to describe copyleft, but has also been used for non-libre licenses (see, for example, theMicrosoft Limited Reciprocal License).
Free content and software licenses without the share-alike requirement are described aspermissive licenses.
As with all six licenses in the current Creative Commons suite, CC Attribution-ShareAlike and CC Attribution-NonCommercial-ShareAlike requireattribution. According to Creative Commons, the advantage of this license is that future users are not able to add new restrictions to a derivative of your work; their derivatives must be licensed the same way.[3]
The 3.0 and 4.0 version of the ShareAlike licenses include acompatibility clause, allowing Creative Commons to declare other licenses as compatible and therefore derivatives may use these instead of the license of the original work.
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Over the years, Creative Commons has issued 5 versions of the BY-SA and BY-NC-SA licenses (1.0, 2.0, 2.5, 3.0 and 4.0).