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SESK: Stuff Everyone Should Know · sesk.info/tech-copyleft-gpl

What is the key idea of a copyleft license like the GNU GPL?

Distributed modified versions must preserve the same freedoms.

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  1. Distributed changed versions must keep the same freedoms ✔
  2. Nobody may sell the software
  3. The code may only be used by nonprofits

Why: Copyleft uses copyright law to keep software free. If you distribute a changed version of GPL code, you must pass on the same freedoms, including the source code. Selling it is allowed.

Huh, didn't know that: The name is a play on copyright: instead of restricting copies, it guarantees the freedoms travel with them.

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My code is under copyleft. I tried copyright, but it felt wrong.
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References

Answer: Distributed changed versions must keep the same freedoms. Copyleft uses copyright law to keep software free. If you distribute a changed version of GPL code, you must pass on the same freedoms, including the source code. Selling it is allowed.

  1. GNU Project: What is Copyleft? www.gnu.org/licenses/copyleft.en.html
    requiring all modified and extended versions of the program to be free as well.
    Checked 2026-10-10.

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