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Wikipedia licenses most of their text and images under two licenses, CC-BY-SA and GFDL. We can choose either of the licenses while using the work.

I know both of them are copyleft licenses and similar in some ways but fail to understand how they differ?

For example, if I use a CC-BY-SA licensed image for my website I have credit the author (with link) and leave a link to the license text. How would it work if I chose GFDL?

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The GFDL was written with books or multi-page documents in mind. It contains many provisions that are cumbersome for non-written works or short written works. It is historically relevant because it is a non-software copyleft license that predates the Creative Commons ShareAlike license.

In nearly all circumstances you will want to choose the CC-BY-SA license over the GFDL: the license is more widely understood, compliance is easier, and (in CC 4.0) it accounts for newer developments like database rights. I could only see the GFDL being preferable if that would allow a user to lawyer around some details of the CC licenses, or if you're actually dealing with a book.

A big difference between CC and GFDL is that GFDL requires all copies to accompanied by a license statement and the full terms of the license. That's practically impossible for images. In contrast, CC allows a link to the full text, an in many cases just the note “CC-BY-SA 3.0” would be sufficient. The GFDL also allows some sections of the document to be marked as invariant, they must be produced verbatim in derived works.

Note that Wikipedia's licensing is a bit more complex. They do use GFDL/CC-BY-SA dual licensing for most text contributions, but should be treated as purely CC-BY-SA licensed by downstream users. Individual pages may contain non-GFDL content. Media files have their own licenses that are noted on the media file's page. Note that the GFDL 1.3 contains an explicit compatibility clause that allowed Wikipedia to relicense their GFDL content to CC-BY-SA in 2009 (but it was scoped to prevent further relicensing).

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  • Does CC BY-SA require source files to be released as GFDL requires? For example, if one is sharing their notes written in PDF format under CC BY-SA, would they require to also release the TeX files used to produce the PDF? As far as I understand, GFDL would require that but I am not sure about CC BY-SA. Thanks!
    – ACat
    Oct 17, 2020 at 10:44
  • @DvijD.C. No, CC licenses do not require “source files” to be released. For example, you can modify a CC licensed image without having to make your Photoshop project available. Someone could also take a CC-BY-SA-licensed TeX file, modify it, and distribute the resulting PDF under the same license, without releasing the modified TeX. In no case (CC, GFDL, GPL, …) would the original author be required to publish source files – they just offer the license but are not bound by its conditions (you can't contract with yourself).
    – amon
    Oct 18, 2020 at 9:49
  • Thanks, the last point is really interesting and weird IMO. I mean if I release a software under GPL without a proper source code, it sort of completely defies the motivation behind GPL. Anyway, if the original author does release the TeX file along with the PDF and release them under GFDL then someone else is required to distribute their modified TeX files when they distribute their modified PDF, right?
    – ACat
    Oct 19, 2020 at 9:26
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    @DvijD.C. The GFDL requires a machine-readable “transparent” copy to be provided depending on the number of copies – the license is mostly concerned about printed material. Would be a good separate question.
    – amon
    Oct 19, 2020 at 10:40
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    Yes, releasing a binary-only software under GPL appears pointless. This has certainly been tried maliciously. But there are also legitimate scenarios, e.g. if the original source is no longer available. GPL doesn't actually require source code, but the “preferred form for making modifications”. Some newer licenses make the downstream recipients a third party beneficiary of the license, which might make it easier to ensure that all parties honour their obligations.
    – amon
    Oct 19, 2020 at 10:41

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