Richard Stallman has published a new guide on gnu.org titled License compatibility and relicensing. Gnu.org is home to a whole host of resources on free software licensing, including frequently asked questions about GNU licenses and our list of free software licenses. Our license list contains information on which licenses are compatible with the GNU General Public License as well as a brief description of what it means to be compatible. This latest article by Stallman provides a more in–depth explanation of what compatibility means and the different ways in which it is achieved.
What is licensing? Why does it matter? Why should you care? There are many reasons that licensing is an important part of a project you are working on. You are taking the time to write code and share it with the world in an open way, such as publishing it on GitHub, Bitbucket, or any number of other code-hosting services. Anyone might stumble across your code and find it useful.
Licensing is the way that you can control exactly how someone who finds your code can use it and in what ways.