Doing my own reading and reasoning about releasing technical content [1] under a free license, I have discovered there are these general approaches:

  • CC-BY-SA only (e.g. StackExchange)
  • FDL only (e.g. Free Software Directory) [2]
  • both CC-BY-SA & FDL (e.g. Wikipedia)

Choosing one of the "only" streets means your work can only be used in similarly licensed works. Choose the "&" option means your work can be used in any of the three.

Has SE explained why they picked CC-BY-SA only, rather than dual-licensing? (This is a question, not a challenge or a suggestion.)

[1] Including software manuals, but not code [3], but including cooking and golfing and maths and stuff cause that's all technical right? :D

[2] The Free Documentation License is a copyleft content license like CC-BY-SA, but older (conceived at the same time as the more famous GPL license for code). Unfortunately you can't freely copy content between FDL and CC-BY-SA works, so the current solution (if you want to make that possible) is to dual-license. (There is a more elaborate backstory [4] here, and as far as I understand the authors of both licenses would like to see a day when they are compatible, it's just not currently possible.)

[3] The inevitable code snippet licensing question, which I finally found an official SE position on.

[4] There appear to be fan clubs in fiercely in favour of particular approaches, and opposed to others. I hope this question doesn't trigger those sorts of responses, I really just want to know why SE chose not to take the dual-licensing route. A suggestion to change would be a separate post.

  • 4
    Speaking as someone who was heavily involved in Wikipedia's licensing, the implementation requirements in the Gnu FDL are a royal pain for any printed work smaller than a software manual. For example, a one-page printout of a GFDL question and answer would require adding six pages of license statement at the end. The only reason Wikipedia is dual-licensed is because it's older than Creative Commons.
    – Mark
    Sep 6 '14 at 1:07

Based on comments from @Mark and some further reading, it seems that the Wikipedia dual license is an accident of history,[1] not a typical "dual license" and not a common practice.

So, it appears that Stack Exchange simply picked the most popular copyleft content license at the time (CC-BY-SA 2.5, since updated to 3.0,[2] and beyond (?)).

If future versions of BY-SA and FDL are declared compatible,[3] dual licensing will not be required thanks to existing mechanisms in BY-SA, and at that point BY-SA and FDL text would be interoperable.

[1] Possibly inaccurate summary: Wikipedia was created before CC licenses existed so used FDL 1.2 "or later", then switched to BY-SA via a legal mechanism introduced explicitly for this purpose in FDL 1.3. As part of this changeover, Wikipedia no longer accepts FDL input, only PD and BY-SA, and only pages with no BY-SA content are still available under FDL. (see "Reusing Wikipedia content")

[2] And fixed the footers but not the legal page (yet)

[3] Possibly by the introduction of "FDLv2" and "Simplified FDL v2", which would allow a path for FDL "with no invariant sections" content to avoid the current incompatibilities with CC-BY-SA. I haven't found anything dated later than 2006 regarding this approach, though.

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