The GNU Free Documentation License (GFDL) is the Free Software Foundation’s copyleft license for written works: software manuals, textbooks, reference material, and other documentation. It was created to solve a gap left by the GNU General Public License, which is written for source code. The FSF recognized that free software needs free documentation, and that a manual locked under restrictive copyright is as much an obstacle to user freedom as proprietary code. The GFDL applies the same copyleft logic to prose that the GPL applies to programs.
The license is explicit about its purpose and its copyleft character. Its preamble states that the goal is to make a document “free” in the sense that everyone has “the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially.” It then defines its mechanism in the same terms the GPL uses: “This License is a kind of copyleft, which means that derivative works of the document must themselves be free in the same sense.” A second aim stated in the preamble is to preserve “for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.”
The GFDL introduced concepts specific to written works that have no real counterpart in software licenses, such as “Invariant Sections”, portions of a document, like a statement of political or philosophical position, that downstream authors must keep unchanged, along with required “Cover Texts” and “Transparent” versus “Opaque” copies that distinguish editable source formats from final rendered output. These provisions were intended to let authors protect an essay or endorsement attached to a manual while still freeing the technical content for reuse, though the invariant-section rules later drew criticism for limiting how freely some documents could be remixed.
The most historically significant deployment of the GFDL was Wikipedia. The encyclopedia was originally released under the GFDL, which made it legal for anyone to copy, mirror, and modify its articles as long as they kept the same freedoms intact. As Wikipedia grew, the friction between the GFDL’s document-oriented design and the realities of collaborative, constantly-edited content became clear, and in time the project migrated its text to the Creative Commons Attribution-ShareAlike license, a transition made possible by deliberate coordination between the FSF and Creative Commons.
The version most commonly referenced today is GFDL 1.3, published on November 3, 2008, with the license itself dating to the original 1.x releases around the turn of the millennium. The GFDL remains part of the FSF’s family of free licenses and is still used for software documentation and some books, standing as the documentation counterpart to the GPL and an early proof that the copyleft idea could protect freedom in writing as well as in code.