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Copyleft or libre share-alike licenses are the largest subcategory of share-alike licenses. They include both free content licenses like Creative Commons Attribution-ShareAlike and free software licenses like the GNU General Public License .
In 1984 or 1985, programmer Don Hopkins sent Richard Stallman a letter labeled "Copyleft—all rights reversed". Stallman chose the phrase to identify his free software method of distribution. [ 4 ] It is often accompanied by a reversed version of the copyright symbol. [ 5 ]
The strength of the copyleft license governing a work is determined by the extent to which its provisions can be imposed on all kinds of derivative works. Thus, the term "weak copyleft" refers to licenses where not all derivative works inherit the copyleft license; whether a derivative work inherits or not often depends on how it was derived.
Partial copyleft exempts some parts of the work from the copyleft provisions, thus permitting distribution of some modifications under terms other than the copyleft license, or in some other way does not impose all the principles of copylefting on the work. For example, the GPL linking exception made for some software packages (see below).
Note: this category differs substantially from Category:Free and open-source software licenses in that it is not limited to software, and not all free software licenses are copyleft (some are permissive, like those of BSD and MIT).
In the mid-1980s, the GNU project produced copyleft free-software licenses for each of its software packages. An early such license (the "GNU Emacs Copying Permission Notice") was used for GNU Emacs in 1985, [5] which was revised into the "GNU Emacs General Public License" in late 1985, and clarified in March 1987 and February 1988.