Re:MySQL man pages silently relicensed away from GPL
by foobarbazqux on Wednesday June 19, 2013 @04:04AM
(#5904281)
Some more examples: the LGPL is copyleft and you can convert it to the GPL. Sometimes the GPL comes with a special exception, which can also be removed. AGPLv3 is more restrictive than GPLv3, and again you can convert GPLv3 to AGPLv3.
If you consider what developers are allowed to do with a piece of software, the main element of copyleft is actually not being able to relicense under a less restrictive license on the spectrum of FOSS licenses. The least restrictive "license" of all is no license, no copyright, i.e. public domain (in the US).
Because you can convert MIT/BSD to a closed source license, but you cannot convert GPL to a closed source license, in this sense the GPL is more restrictive than MIT/BSD. You can convert MIT/BSD to GPL (less to more) but not GPL to MIT/BSD (more to less).
I understand that closed source licenses are the most restrictive of all, this is just an alternative perspective on software freedom with regard to developers.
Not being able to relicense under a license not acceptable to the authors of the particular copyleft license at issue is a main element of copyleft, but copyleft licenses often allow redistributing under more restrictive licenses that remain acceptable to the copyleft license author (e.g., the FSF encourages the use of "or any later version" language in GPL licenses, and GPLv3 is in many respects more restrictive than GPLv2; further, GPLv3 allows relicensing with specified kinds of additional terms that may in some ways be more restrictive.)