The GNU Free Documentation License (GFDL or FDL), first released in March 2000, applies the principles of copyleft to documentation, manuals, and other reference works. It was created by stallman and the free-software-foundation to address a gap: the gpl-v2 was designed for software source code, but the gnu-project needed a license for manuals, tutorials, and reference works that would guarantee the same freedoms — to read, copy, modify, and redistribute — while accommodating the specific characteristics of documentation.
Documentation differs from software in legally and practically significant ways. A software manual may contain "invariant sections" — statements of the author's position, dedications, or historical background — that the author wants preserved verbatim even as the technical content is updated. The GFDL accommodates this by allowing the licensor to designate invariant sections that must be retained without modification, and front- and back-cover texts that must appear on printed versions. These provisions are more restrictive than the GPL and generated significant controversy.
The most consequential use of the GFDL was Wikipedia's adoption of it as the initial license for all content from the project's founding in 2001. This applied the copyleft principle to a collaborative encyclopedia, ensuring that Wikipedia's content could not be incorporated into proprietary products without sharing alike. In 2009, the Wikipedia Foundation and free-software-foundation worked out a migration path that allowed Wikipedia to dual-license under GFDL and Creative Commons Attribution-ShareAlike 3.0. larry-lessig's creative-commons had developed CC-BY-SA as a more documentation-friendly copyleft license, and the migration enabled broader reuse. stallman supported the migration while maintaining that the GFDL remains the appropriate choice for software documentation.
The GFDL's "invariant sections" provision has been criticized — including by Debian's project policy team — as incompatible with the Debian Free Software Guidelines and the open-source-initiative's open source definition. Debian classified GFDL-licensed documentation with invariant sections as non-free for a period. The controversy illustrates the tension between stallman's conviction that documentation should carry political and philosophical content (his own GNU manuals typically contain statements of the free software philosophy) and the broader free software and open source community's preference for minimally restrictive licenses.
The gnu-emacs-manual is one of the most prominent works licensed under the GFDL, and its invariant sections include Stallman's philosophical statements about software freedom.