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Open-source software (OSS) license compatibility is part of each license term which regulates an allowance for the usage of the source code previously released under license A into the new project which has the license B. This means that license A is compatible with license B. Roughly speaking (it is not a strong rule and depends on each license terms), permissive licenses are compatible with protective ones. Vice versa compatibility is usually not the case.
The GNU General Public License (GPL) is the most popular example of copyleft (protective) licenses. Due to its protective nature, it has a cumulative effect: the amount of software with such license increases every year. As a result, there is often a practical need to use the GPL-released portion of the code as a part of a new or, which is more complex, an existing project already released under a different OSS license. Nevertheless, the flexibility and simplicity of the permissive licenses lead to an increasing trend in their usage over the last years.
Care must be taken even during a combination of intellectual property products released under different GPL-family licenses and their versions. The scale from less to more protective GPL licenses is described in the diagram below. 
GPL compatibility
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