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I would like to ask whether an otherwise standard 3-clause BSD license would remain GPL compatible if the following 4th clause is added:

  1. Altered versions (source code and/or binary) must be plainly marked as such, and must not be misrepresented as being the original software.
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Yes, such a license would be compatible with GPL-3.0. In particular, the GPL already has the following terms:

  • Section 5(a) is always required:

    The work must carry prominent notices stating that you modified it, and giving a relevant date.

  • The GPL-3.0 allows certain additional terms to be added to the license. One of these is section 7(c):

    Prohibiting misrepresentation of the origin of that material, or requiring that modified versions of such material be marked in reasonable ways as different from the original version

But if you're looking for a permissive license with such terms that is GPL-3.0 compatible, consider just using the Apache-2.0 license.

  • Section 4(b):

    You must cause any modified files to carry prominent notices stating that You changed the files

  • Misrepresentation of origin is forbidden implicitly through section 6, but it's only enforceable if you have a trademark:

    This License does not grant permission to use the trade names, trademarks, service marks, or product names of the Licensor, except as required for reasonable and customary use in describing the origin of the Work and reproducing the content of the NOTICE file.

If you create a custom license, please give it a custom name, and do not call it “BSD-something”. This also makes it easier for people to understand that your license would not have been FSF- or OSI-approved (even though I'm sure that it is compatible with the Free Software Definition and the Open Source Definition).

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  • I don't believe this is quite correct. Section 5(a) of GPL-3.0 is a limitation on conveyance of the work only. His proposed clause (the first part, not the second) is a limitation on modification of the work whether conveyed or not. I don't see why that wouldn't be a sufficient difference to make his license not compatiable with the GPL because it is an additional restriction.
    – David Schwartz
    Nov 30, 2022 at 14:54
  • @DavidSchwartz That is an excellent point. I didn't notice that OP's clause, as phrased, would apply to unpublished modifications. However, I'm not sure if that subtlety was OP's intention.
    – amon
    Nov 30, 2022 at 15:49
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    Probably not. The point is that the OP can get what they want without breaking GPL compatibility as long as the wording is careful not to go beyond what the GPL requires. But this does point out how dangerous it is to try to craft your own license. Not only can little details bite you but you also have the problem of convincing others that they can rely on it.
    – David Schwartz
    Nov 30, 2022 at 16:29
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    @DavidSchwartz The clause is mostly taken from the zlib license, which is GPL compatible. The main difference is that the zlib license only requires modified source code to include modification notices while I require both modified source and binary distributions to include it as well.
    – user28612
    Dec 1, 2022 at 9:57
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    I asked my own question.
    – David Schwartz
    Dec 1, 2022 at 21:56

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