GNU Free Documentation License

The GNU Free Documentation License is a free content license for software documentation written by Richard Stallman and advocated by the Free Software Foundation (FSF).

It is unremittingly awful and grossly unsuitable for anything other than software manuals written on paper, and often not even those.

It is a cautionary tale in the dangers of excess restraint in the name of freedom.

Cryptographic clarity
No one understands how to reuse GFDL content safely, including the FSF. In the context of mirroring a widely-edited wiki or a page thereof, its terms are difficult to follow, legally unclear and may be technically impossible to comply with in a comparable degree of safety to the GPL or CC BY-SA. And don't even talk about images. Before Wikimedia went Creative Commons, tedious nerds would frequently claim that any given reuser of Wikipedia content was technically violating the GFDL no matter what shrubberies they obtained (thus putting off quite a lot of reusers).

The Debian project classes it as a "non-free" license because its terms are unclear and onerous in practice. The Debian Free Software Guidelines threat model is: if an innocent reuser follows the terms, can an insane copyright holder sue them anyway and have it not thrown out immediately? The GFDL failed in several regards. Some GFDL variants are allowed as technically DFSG-free, but Debian still recommends against it as it's incompatible with everything else on Earth.

The only reason it is not a minor footnote in the "miserable failures" section of free licensing history is that it was used for Nupedia (because the Creative Commons (CC BY-SA) hadn't started then), which evolved into Wikipedia, which also used it, and quickly became the largest repository of GFDL content by far. With many years' effort, the Wikimedia Foundation convinced the FSF and Stallman in 2008 to allow relicensing GFDL content on wikis as CC BY-SA, so that text could be sanely edited on a wiki, let alone shared with the entire rest of the free-content world, who had gone for a license easily obeyed by mere humans.

There were those who vociferously objected to Wikimedia changing from GFDL to CC BY-SA, because they considered the difficulty of reusing content in any practical manner to be a feature rather than a problem. They may have rather missed the point.

Appallingly, the FSF is still pushing the thing, even though they themselves say at the tricky bits "better get a lawyer, son." Because freedom only for those with copyright lawyers on retainer is still technically freedom, right? Unless you have a spectacularly good reason, ignore them and just use CC BY-SA, CC BY or public domain, like everyone else does.

The GFDL is not to be confused with the GNU Public License (GPL), a far better license intended specifically for software.

The good points

 * The escape clause (section 11).
 * Providing thought experiments for bored copyright nerds.
 * Um ...
 * That's it.

In fairness it was one of the first "copyleft" licenses for data (though the Design Science License may predate it ), so it's understandable that significant mistakes were made. That it managed to fit so many mistakes into one license at least provided a warning for those who were to follow.