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I am developing a program from another program (a fork) under GPLv3. I know that if "my" program is used commercially or just distributed (as opposed to just personal use) it should keep the same license.

I don't understand from the GPLv3 license terms, whether this program which would be available open source, can be used as a piece inside proprietary software or not.

Can someone let me know and maybe tell which part of the license states this?

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  • It's in Section 0, Definitions: To "modify" a work means to copy from or adapt all or part of the work in a fashion requiring copyright permission, other than the making of an exact copy. The resulting work is called a "modified version" of the earlier work or a work "based on" the earlier work.. So, according to the GPL, including even small portions of the work (assuming those small portions receive copyright protection) in your software makes your software "based on" the GPL software. And the GPL requires you to make your software available under the GPL as well in that case.
    – Brandin
    Oct 27, 2022 at 12:37
  • @Brandin I think the "based on" or "derived work" is actually true in the USA due to copyright law alone, without GPL involved. Of course the GPL reminding you means you have no excuse to say "I didn't know this" to reduce damages.
    – gnasher729
    Mar 11, 2023 at 1:21
  • Thank you @Brandin
    – Mah Neh
    Mar 11, 2023 at 11:04

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If proprietary software contains GPL-licensed software then the same rules apply. Whether it's 99% GPL-licensed with 1% changes, or 99% proprietary with 1% GPL-licensed code added, makes no legal difference.

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  • Do you mean that it should be open source etc and be fully re-licensed to GPL ? Where do you read that ?
    – Mah Neh
    Oct 21, 2022 at 18:17
  • A (larger) product to which you add GPL licensed code will become a derived work, unless you keep both works separate, which will be difficult.
    – gnasher729
    Oct 22, 2022 at 18:33
  • @MahNeh In the GPL v3, see section 5, "Conveying Modified Source Versions." The relevant section says "You may convey a work based on the Program ..." and then requires that "the work must carry prominent notices stating that it is released under this License." In the definitions section, "This License" with capital L is defined as referring to the GPL version 3. And as mentioned elsewhere, "a work based on the program" is also defined in the definitions as basically any sort of use of the code other than making an exact copy.
    – Brandin
    Mar 23, 2023 at 13:09

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