The right to fork describes an effort, relating to an open source project, to break off and to steer the project in a direction that seems to be more appropriate. Either projects continue in parallel, but to different aims.
A good example might be the EGCS fork from GCC. One group has thought that the project would be better served if it moved forward faster, and since the project is open-sourced, they have every right to take the code and continue working on it to fit their needs. Nowadays, EGCS has replaced GCC in newer versions.
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