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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 open content counterpart to the GNU GPL. The current state of the license is version 1.2, the official text of which can be found here.
The license was designed for manuals, textbooks, and other reference and instructional materials. 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.
About the GFDL Edit
Secondary Sections Edit
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.
Using the GFDL Edit
Materials for which commercial redistribution is prohibited Edit
Materials for which commercial redistribution is prohibited generally cannot be used in a GFDL-licensed document, e.g., a Wikipedia article, because the license does not exclude commercial re-use. However in some specific cases, commercial re-uses may be fair use and in that case such materials 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.
Many people and groups, notably the Debian project (based on their Debian Free Software Guidelines), 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.
A number of objections have been made to the GNU FDL, with some critics suggesting to use other licenses, such as the Creative Commons license, or even using 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 are:
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 you are not allowed to save document copies you 'make' 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."
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.
GPL incompatible in both directionsEdit
The GNU FDL is incompatible in both directions with the GPL: that is GNU FDL material cannot be put into a GPL program, and text from a GNU program cannot be put into the GFDL. Because of this, code samples are often dual-licensed so that they may appear in documentation.
Burdens when printingEdit
The GNU FDL requires that when printing out a document covered under this license you must also include: "this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies". What this means is that if you print out a copy of an article on Wikipedia you must also include a copyright notice and a physical print out of the GNU FDL.