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. AGPLv

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.