May 13, 2008
When a program is free but not copylefted, some copies or modified versions may actually not be free. Non-copylefted free software comes from the author with permission to redistribute and modify while also adding restrictions to it. A software company can compile the program and distribute the executable file as a proprietary software product, with or without modifications.
The GNU Project from which Linux comes from promotes copylefting of almost all the written software to ensure that every user gets all the freedom implied by the term “free software”. There are many possible ways of writing copyleft distribution terms thus there can be many copyleft free software licenses. However, different copy licenses are usually incompatible.
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