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Copyleft is a distinguishing feature of some free software licenses, while other free-software licenses are not copyleft licenses because they do not require the licensee to distribute derivative works under the same license. There is an ongoing debate as to which class of license provides the greater degree of freedom.
The GNU General Public Licenses (GNU GPL, or simply GPL) are a series of widely used free software licenses, or copyleft licenses, that guarantee end users the freedoms to run, study, share, and/or modify the software. [7] The GPL was the first copyleft license available for general use.
License compatibility is a legal framework that allows for pieces of software with different software licenses to be distributed together. The need for such a framework arises because the different licenses can contain contradictory requirements, rendering it impossible to legally combine source code from separately-licensed software in order to create and publish a new program.
Permissive licenses come from academia. Copyleft licenses come from the free software movement. Copyleft licenses require derivative works to be distributed with the source code and under a similar license. Since the mid-2000s, courts in multiple countries have upheld the terms of both types of license.
Copyleft licenses (also known as "share-alike"), [46] require source code to be distributed with software and require the source code be made available under a similar license. [48] [49] Copyleft represents the farthest that reuse can be restricted while still being considered free software. [50]
These licenses have been described pejoratively as viral licenses, because the inclusion of copyleft material in a larger work typically requires the entire work to be made copyleft. The term reciprocal license has also been used to describe copyleft, but has also been used for non-libre licenses (see, for example, the Microsoft Limited ...
Some licenses are called "copyleft" licenses. Essentially, they have three key properties: A work licensed with a copyleft license can be copied at will. All published derivative works must use exactly the same license as the original: if you use the work, you're forced to use the same license for your own original work as well.
The strength of the copyleft governing a work is an expression of the extent that the copyleft provisions can be efficiently imposed on all kinds of derived works. "Weak copyleft" refers to licenses where not all derived works inherit the copyleft license; whether a derived work inherits or not often depends on the manner in which it was derived.