The GNU Free Documentation License (GNU FDL or simply GFDL) is a copyleft license for free content, designed by the Free Software Foundation (FSF) for the GNU project. It is the counterpart to the GNU GPL. The current state of the license is version 1.2.
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. It stipulates that any copy of the material, even if modified, carry the same license. Those copies may be sold but, if produced in quantity, have to be made available in a format which facilitates further editing.
Projects under GFDL include Wikipedia, which is the largest documentation project among them.
About the GFDL
The license explicitly separates any kind of "Document" from "Secondary Sections", which may not be integrated with the Document, but exist as front-matter materials or appendices. Secondary sections can contain information regarding the author's or publisher's relationship to the subject matter, but not any subject matter itself. While the Document itself is wholly editable, and is essentially covered by a license equivalent to (but both-ways incompatible with) the GNU General Public License, some of the secondary sections have various restrictions designed primarily to deal with proper attribution to previous authors.
Specifically, the authors of prior versions have to be acknowledged and certain "invariant sections" specified by the original author and dealing with his or her relationship to the subject matter may not be changed. If the material is modified, its title has to be changed (unless the prior authors give permission to retain the title). The license also has provisions for the handling of front-cover and back-cover texts of books, as well as for "History", "Acknowledgements", "Dedications" and "Endorsements" sections.
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 incompabitible with the license and cannot be incorporated into the work. However, incorporating such restricted material may be fair use and do not need to be licensed to fall within the GFDL if such fair use is covered by all potential subsequent uses. One good example of such liberal and commercial fair use is parody.
Criticisms of the GFDL
Many people and groups consider the GFDL a non-free license. The reasons for this are that the GFDL allows "invariant" text which cannot be modified or removed, and that its prohibition against digital rights management (DRM) systems affects valid usages as well. Some members of the Debian project (based on their Debian Free Software Guidelines) agree. The project eventually voted  to consider works licensed under the GFDL to be free provided the invariant section clauses are not used.
A number of objections have been made to the GNU FDL, with some critics recommending the use of alternate licenses (such as the share-alike Creative Commons licenses) or even the GNU GPL. The Debian project has a detailed draft of objections and Nathanael Nerode has also  summarized his objections. Often mentioned arguments against the GFDL include:
Overly broad DRM clause
The GNU FDL contains the following statement.
- "You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute."
A criticism of this language is that it is too broad, because it applies to private copies made but not distributed. This means that a licensee is not allowed to save document copies "made" in a proprietary file format or using encryption.
- "This means that you cannot publish them under DRM systems to restrict the possessors of the copies. It isn't supposed to refer to use of encryption or file access control on your own copy. I will talk with our lawyer and see if that sentence needs to be clarified."
As of 2006, the sentence has not yet been clarified.
A GNU FDL work can quickly be encumbered because a new, different, title must be given and a list of previous titles must be kept. This could lead to the situation where there are a whole series of title pages, and dedications, in each and every copy of the book if it has a long lineage. These pages cannot ever be removed, at least not until the work enters the public domain after copyright expires.
- "The goal of invariant sections, ever since the 80s when we first made the GNU Manifesto an invariant section in the Emacs Manual, was to make sure they could not be removed. Specifically, to make sure that distributors of Emacs that also distribute non-free software could not remove the statements of our philosophy, which they might think of doing because those statements criticize their actions."
Ironically, this problem is reminiscent of GNU's problem with the 4 clause BSD license.
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.
Burdens when printing
The GNU FDL requires that licensees, when printing a document covered by the license, must also include "this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies". This means that if a licensee prints out a copy of an article whose text is covered under the GNU FDL, he or she must also include a copyright notice and a physical printout of the GNU FDL, which is a significantly large document in itself.
The license has a preamble, which some critics dislike because of its ideological tone. Because the preamble is part of the license, it must be included (along with the rest of the license's text) with every copy of a licensed document.
Some consider the license to be too long.
The definition of a "transparent" format is complicated, and may be difficult to apply. For example, drawings are required to be in a format that allows them to be revised straightforwardly with "some widely available drawing editor." The definition of "widely available" may be difficult to interpret, and may change over time, since, e.g., the open-source Inkscape editor is rapidly maturing, but is still in a prerelease stage. This section, which was rewritten somewhat between versions 1.1 and 1.2 of the license, uses the terms "widely available" and "proprietary" inconsistently and without defining them. According to a strict interpretation of the license, the references to "generic text editors" could be interpreted as ruling out a format used by an open-source word-processor such as Abiword; according to a loose interpretation, however, Microsoft Word .doc format could qualify as transparent, since a subset of .doc files can be edited perfectly using OpenOffice.org, and the format therefore is not one "that can be read and edited only by proprietary word processors."
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.
- The Creative Commons "CC-by-sa" and "CC-nc-sa" licenses
- Design Science License
- Open Content License
- Open Publication License
- BSD license
- Free software license
- Open content
- Open source hardware
- Software licensing
- GFDL official text
- Listen to the GFDL (file info) —
- Audio recording of the full text of the GNU Free Documentation License.
- Problems listening to the file? See media help.
Resources discussing the appropriateness of the GFDL:
- The GNU Free Documentation License
- Draft of Debian position statement about the GFDL
- Why You Shouldn't Use the GNU FDL
- Why Wikitravel isn't GFDL: Problems with using the GFDL for short printed texts
- The Free Universal Encyclopedia And Learning Resourcear:رخصة الوثائق الحرة (جنو)
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- Listen to the GFDL (file info) —