GNU Free Documentation License

The GNU Free Documentation License (GNU FDL or simply GFDL) is a copyleft license for free documentation, designed by the Free Software Foundation (FSF) for the GNU project. It is the counterpart to the GNU GPL that gives readers the same rights to copy, redistribute, and modify a work and requires all copies and derivatives to be available under the same license. Copies may also be sold commercially, but if produced in larger quantities (greater than 100) then the original document or source code must be made available to the work's recipient.

The license was designed for manuals, textbooks, other reference and instructional materials, and documentation which often accompanies GPL software. However, it can be used for any text-based work, regardless of subject matter.

Commercial redistribution
The GFDL requires the ability to "copy and distribute the Document in any medium, either commercially or noncommercially" and therefore is incompatible with material that excludes commercial re-use. Material that restricts commercial re-use is incompatible with the license and cannot be incorporated into the work. However, incorporating such restricted material may be fair use under United States copyright law and does not need to be licensed to fall within the GFDL if such fair use is covered by all potential subsequent uses. One example of such liberal and commercial fair use is parody. Note that a "fair use" in the United States might well be considered an infringing use under the copyright laws of other countries.

GPL incompatible in both directions
The GNU FDL is incompatible in both directions with the GPL: that is GNU FDL material cannot be put into GPL code and GPL code cannot be put into a GNU FDL manual. Because of this, code samples are often dual-licensed| so that they may appear in documentation and can be incorporated into a free software program.

Other free content licenses
Some of these were developed independently of the GNU FDL, while others were developed in response to perceived flaws in the GNU FDL.
 * FreeBSD Documentation License
 * Creative Commons licenses
 * Design Science License
 * Free Art license
 * Open Content License
 * Open Publication License