Compatibility is not a legal term. It is, however, an issue raised from the nature of software development. Software development often involves more than one program and the resulted project makes use of various programs or modules. If the design simply involves the linking of two programs, there are usually no compatibility issues. But if the design involves the integration of two modules into a new program, we will have to investigate if the respective license of each program is compatible with each other. The rule of thumb is, if the licenses of the two programs have no conflicts in terms of rights of obligations, it is then very probable that the two licenses are compatible with each other. On the other hand, if they have conflicting terms on rights and obligations, the two programs may not be able to be put together. For further information on compatibility, the explanation on the GNU Project’s website can also be helpful: if program A is licensed under license A whereas program B under license B’, if you want to combine A and B into a larger program C, C will become the derivative work of both A and B, and the license chosen for C will depend on the terms of A’ and B’. If license A’ does not restrict the combination of A and B and if license B’ does not restrict the combination of B and A, plus there are no conflicting terms, we say licenses A’ and B’ are compatible and programs A and B can be combined. Although program C may adopt a license other than A’ or B’, the terms of license C’ may not be in conflict with the terms of licenses A’ and B’. The website of the GNU Project lists a number of licenses that are compatible with the GPL and also a number of licenses that are not compatible with the GPL. Please refer to: https://www.gnu.org/philosophy/license-list.html This website also offers an article that discusses the compatibility of different licenses. Please refer to “Compatibility of Free Software Licenses”.