In order to protect these freedoms, software licenses have been developed, to ensure that the software is able to meet the "Libre" qualifications. The license also protects these programs from becoming proprietary, or commercial, whereby the source code becomes closed off from the user.
Licenses have also been developed for Libre Documentation, most notably the GNU Free Documentation License, or FDL. Other documentation licenses include the FreeBSD Documentation License, Apple's Common Documentation License, and the Open Publication License, however it must be stated that some are restrictive and not compatible with the GNU FDL.
The GNU Free Documentation License states its purpose clearly in the preamble, which is quoted below:
0. PREAMBLE
The purpose of this License is to make a manual, textbook, or other written document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and the publisher a way to get credit for their work, while not being considered responsible for modifications made by others. This license is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software. We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
The GNU FDL is included in its entirety at the end of this article.