Share-alike

A share-alike license for software or documentation requires that any improvements made by any user be shared on like terms (i.e. the same license) with all other users. This was the most controversial term of the original GPL which started the free software movement. Since then, it has become central to the open content movement:


 * The Creative Commons suite of licenses today includes a range of share alike licenses usually denoted with "sa" in their name, e.g. the CC-by-sa requires attribution and share-alike redistribution, while the CC-nc-sa requires non-commercial use only without further payment or permission. Such licenses are widely used in the large public wikis and alternative media on the Internet.


 * The GFDL is also a share-alike license, considered closely equivalent to the CC-by-sa. Some services permit contributions under more than one license, e.g. Wikinfo, which accepts both GFDL and CC-licensed open content.