It's also important to note that open source licences only *give* rights to others; the only limitation it places on the copyright holder is that they cannot revoke those rights.
That's why even under the GPL, the copyright holder can still build a closed product on top of it.
It's not just to enable closed source work though. It can also prevent having to reach out to all contributors if there's a good reason to change the licence in the future.
For example, this is what LLVM is having to do now: https://foundation.llvm.org/docs/relicensing_long_tail/
@humanetech Yeah, I was thinking about adding something about the DCO, but decided that would be wading too far into the weeds.
To be clear, the DCO solves a different problem than the one LLVM is running into now: to ensure that those who licence their work take responsibility for being allowed to do so (i.e. that they're the copyright holders). It does not allow an organisation to later re-license that work. (Which, of course, is exactly the reason for going with a DCO over a CLA.)
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