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The GNU Free Documentation License (GFDL) is a copyleft license for free content, designed by the Free Software Foundation (FSF) for the GNU project. The current state of the license is version 1.3. See "GNU Free Documentation License 1.2" and "1.3" to view the current versions.
The license is designed for software documentation and other reference and instructional materials. 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.
For further information, consult the Wikipedia article on the GNU Free Documentation License or visit the GFDL web page.