How to Use the Optional Features of the GFDL

The GNU FDL (GNU Free Documentation License) includes two optional features, invariant sections and cover texts, which you can use if you wish. The manual's license notice should say whether you are using these features.

The simplest case is that you do not use these features. Then the license notice should say so, like this:

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".

But if you want to use these features, here is an explanation of what they are intended for and how to use them.

Invariant sections

The idea of invariant sections is that they give you a way to express nontechnical personal opinions about the topic.

The classical example of an invariant nontechnical section in a free manual is the GNU Manifesto, which is included in the GNU Emacs Manual. The GNU Manifesto says nothing about how to edit with Emacs, but it explains the reason why I wrote GNU Emacs—to be an essential part of the GNU operating system, which would give computer users freedom to cooperate in a community. Since the GNU Manifesto presents the principles of the GNU Project, rather than features of GNU Emacs, we decided that others should not remove or change it when redistributing the Emacs Manual, and we wrote that requirement into the license. In effect, we made the GNU Manifesto into an invariant section, though without using that term. To do this today using the GNU FDL, we would write the license notice like this:

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being just “GNU Manifesto,” with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License.”

This says that the section “GNU Manifesto” is the only invariant section.

The old Emacs Manual license did not allow others who modify and redistribute the manual to add new invariant sections. But while formulating a more general version of the concept for the GNU FDL, I felt that it would be more morally consistent if those who modify a manual can write invariant sections, just as the first authors can. The FDL permits this.

But that feature requires a safeguard to prevent it from being abused to endanger the free status of the manual. Person B who modifies a manual that was written by person A should not be able to make any of person A's documentation invariant, for that would deny person C the permission to modify it further. Likewise, if person B adds documentation for additional subtopics, this added documentation must not be invariant; the documentation itself must be modifiable by others. The FDL's conditions on adding invariant sections provide this safeguard.

There is no need for a manual to have any invariant sections. The simplest case is to have none at all. You do not have to list the GNU FDL itself as an invariant section, because the FDL explicitly says that the FDL itself may not be changed. The FDL also distinguishes certain section titles, “History” and “Dedications” and others. You should not list these sections as invariant, because the FDL already gives the rules for these sections.

Cover texts

A cover text is a short piece of text that you insist must be printed on the cover of the manual when the manual is published, even if someone else is publishing it. You can specify a “front-cover text” which has to be printed on the front cover, and you can specify a “back-cover text” which has to be printed on the back cover. You can specify one of each.

Cover texts are a new feature, so I cannot cite past examples of their use, but the intended purpose is simple. They are meant to give the original publisher of an edition a way to ask or encourage people to buy the copies they publish rather than those of some other reprinter. For example, we could imagine specifying this as a back-cover text:

Free Manuals Inc. paid Alyssa P. Hacker to write this manual and asks for your support through buying the Free Manuals edition.

This might help Free Manuals Inc. succeed in the business of publishing free manuals—at least, that is the plan. To specify this back-cover text, Free Manuals Inc. would write the license notice like this:

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and one Back-Cover Text: “Free Manuals Inc. paid Alyssa P. Hacker to write this manual and asks for your support through buying the Free Manuals edition.” A copy of the license is included in the section entitled “GNU Free Documentation License.”