GPL Compatibility
David A. Wheeler has argued that GPL compatibility is an important feature of software licenses. Many of the most common free software licenses, such as the original MIT/X license, BSD licenses (in the three-clause and two-clause forms, though not the original four-clause form), MPL 2.0, and LGPL, are "GPL-compatible". That is, their code can be combined with a program under the GPL without conflict (the new combination would have the GPL applied to the whole). When it comes to copyleft software licenses, they are not inherently GPL-compatible, but some have exception clauses that allow combining them with software that is under different licenses or license versions.
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