Most GPLv2 licensed software includes the clause &quot;either version 2 of the License, or (at your option) any later version.&quot;  Through this mechanism a lot of GPL licensed software becomes compatible with Apache 2.<p>Also, replying to something a little higher in this thread, I wouldn&#x27;t say that Apache 2 defines all contributions as Apache 2.  That section of the license starts with the words &quot;5. Submission of Contributions. Unless You explicitly state otherwise, ...&quot;<p>And so Apache 2 just becomes the assumed default license on contributions, but it&#x27;s not at all forced or required that contributions come in under Apache 2.