GNU Free Documentation License
The GNU Free Documentation License (GFDL) is promoted by the Free Software Foundation, Inc. as a way supposedly to facilitate the free use and modification of textual information. It is modeled after the approach taken by free software developers to encourage improvements and use of free software, despite fundamental differences between editing and using text rather than software. But in fact its red tape and tricky restrictions have limited its usefulness.
The GFDL license is complex and burdensome, and difficult for teachers to use. It is rare to see a book or educational material that is based on reuse of material provided under this license.