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Open Source and Free Documentation Licenses, Part 1: The GNU FDL

by Andrew M. St. Laurent, author of Understanding Open Source and Free Software Licensing
09/16/2004

Open source and free software licensing is generally associated with software development, and for good reason. The ideas underlying open source licensing were all developed in the context of software creation. The very name "open source" refers to the opening of the source code in which software is written.

Open source principles make sense for software development. Because the source code is "open," users can make modifications to the underlying code, either in the form of incremental improvements or in the form of a total redevelopment that make the program into something entirely new. Code, at root, performs functions: the same code that allows searches in an airline database program may permit purchasers to select individual concert tickets in another program, with relatively little modification. It is the way that open source principles take advantage of the fluidity and power inherent in code that makes open source licensing so powerful.

To take advantage of these open source principles, open source licenses must at a minimum give users access to the source code, the legal ability to distribute the program both as source code and in executable form, and the ability to modify the source code to create new or derivative works which may themselves be freely distributed.

Related Reading

Understanding Open Source and Free Software Licensing
By Andrew M. St. Laurent

These same principles have also been applied to licenses for documentation and works other than software. Because documentation does not have the same fluidity and power that code does, it is an open question whether open source principles will prove to be as revolutionary in connection with such works as they have been with software. Nonetheless, at the very least, such licenses are useful, if for nothing else than the distribution of documentation and manuals related to open source programs.

The licenses discussed in this series of articles--the GNU Free Documentation License (FDL), the Open Publication License, and the Open Gaming License--are directed at documents in particular. They reflect a fundamental split in licensing philosophies associated with different groups of open source licenses. The GNU Free Documentation License, described in this article, applies to documents the same requirements of reciprocity applied by the GNU General Public License to software. That is to say, it requires that derivative works (modified versions of the original work, or works otherwise based on the original work) be licensed only under the GNU FDL. This is the so-called "viral" effect of the GNU system of licenses: works licensed under a GNU license, works derived from those works, works derived in turn from such derivative works, and on and on, can only be licensed under the GNU license under which the original work itself was licensed.

By contrast, the Open Publication License and the Open Gaming License, which will be discussed in the next two articles, follow the path of academic licenses such as the BSD and MIT licenses, at least in some variations, permit derivative works to be distributed under other licenses, including proprietary licenses, so long as certain conditions are complied with, mostly relating to providing notice of the name and publisher and the license applicable to the original work.

The Creative Commons project and the licenses associated with it represents a much more ambitious attempt to apply open source principles to a number of different media, including music, film, and performances, as well as written work. Those licenses are discussed in more detail in my book, Understanding Open Source and Free Software Licenses.

The GNU Free Documentation License

As can be deduced from its title, the GNU Free Documentation License applies the GNU licensing methodology embodied in the GNU General Public License (GPL) and the GNU Lesser General Public License (LGPL) to documentation that accompanies software, although it is applicable to other types of work. Its preamble states its purposes as, first, making "free" (in the sense of readily available for distribution) manuals, textbooks, and other useful written works and, second, assuring credit to the original author of the work. The license quite explicitly attaches the principle of copyleft to the licensed work, permitting the free distribution or modification of the work so long as the distributed or modified work is itself licensed under the GNU FDL.

The version of the license discussed in this article in v1.2, copyright 2002 by the Free Software Foundation. The complete text of the license is attached as Appendix A.

Any written work may fall within the scope of the GNU FDL so long as it has a notice that the license applies to it. The license only applies to copying, modifying, or distributing the work, all acts that would implicate rights created under copyright law. Like the GPL, the FDL does not apply to simple "use" of the work: anyone, for example, can read an GNU-FDL-licensed work without implicating the rights of the copyright holder or falling within the scope of the license. Finally, like the GPL, the FDL permits the distributors to charge money for physical copies--but they cannot limit the rights of those receiving copies to make copies or otherwise exercise their own rights under the FDL.

Although there are some ambiguities, the FDL at root requires that the substantive work itself (called the "Document" by the FDL) be open for modification and distribution. Certain texts, including those that relate only to licensing or to the original author's intended description of the work--either its title, or a brief paragraph intended for the back cover of the printed form of the Document--cannot be modified. These specific texts (which must be specifically identified by the author or copyright holder as texts that cannot be modified) must be distributed unmodified by licensees in connection with their distributions of the Document or modified versions of the Document.

The FDL permits essentially five different actions to be taken in connection with the Document:

  1. Verbatim copying.
  2. Verbatim copying in quantity.
  3. Modifying the document.
  4. Combining or collecting the work with other works issued under the FDL.
  5. Aggregating the work with other works under the FDL or other licenses.

Each of these five methods of distribution relies to varying extents on terms that are defined in Section 1, "Applicability and Definitions."

Section 1 definitions include the "Document," meaning the substantive work, for example a manual on Emacs called the Emacs Helper. A "Modified Version" is any version of the Document, whether translated, modified, or copied only in part. A "Secondary Section" is a piece of text, which is a part of the "Document," that does not address the subject matter of the document but instead deals with the relationship of the FDL to the intentions of the author or copyright holder. For example, the author of the Emacs Helper may want to include a short essay describing why he chose to use the FDL to license the Document. An "Invariant Section" is a Secondary Section that has been designated as "Invariant" and that must be included in redistributed versions. "Cover Texts" are short texts, five words for the Front-Cover Text and twenty five words for the Back-Cover Texts, designated by the original author. These Cover Texts are distinct from the title of the Document, which the FDL addresses separately. A Document need not have any Cover Texts, any Secondary Sections, or any Invariant Sections contained within it.

Section 1 also defines "Transparent" and "Opaque" versions of the Document. A "Transparent" version is the text equivalent of source code as that term is used in the GPL. A Transparent version of the Document is in a machine-readable format, in a file format that is readily modifiable using commonly available text editors; plain ASCII without markup is such a format. Note that a file in a proprietary format such as Microsoft Word is not Transparent under this definition. An "Opaque" version is any other version of the Document, including Documents printed on paper or in other formats not suitable for editing.

Finally, Section 1 permits but does not require warranty disclaimers to apply to the Document. Such disclaimers typically try to protect authors against liability for damages that might flow from the use of the work, or from infringement on the copyrights or other intellectual property rights of other authors.

Section 2 of the FDL is entitled "Verbatim Copying": it permits the most straightforward use of the Document licensed under the FDL. In order to do that, the person copying the Document, the licensee, need only copy the work exactly as it is, including the copyright notice and the license notice saying that the FDL applies to it. It also requires that a copy of the license accompany every such copy: if the author of the Work has not included such a copy, the licensee must attach a copy of the FDL text to the copy. Such copies must be distributed under the terms of the FDL with no additional terms attached. That means that any recipient of the Document has the exact same rights with respect to the Document that the licensee does, including the rights to modify or distribute the Document as provided by the FDL. The licensee can't use technical measures to restrict distribution, such as distributing the work in a way (like a locked PDF file) that would prevent copying or modifying the Work. The licensee need not copy or distribute the Front-Cover or Back-Cover Texts, if any. However, because the Secondary Sections are part of the Document, including any Invariant Sections, they must be included in distributions under Section 2. Similarly, if the Document has warranty disclaimers, the FDL considers those part of the license for distribution purposes. This means that any copy of the Document must have the warranty disclaimers, if any, that were attached to the Document in its original form.

Section 3 of the FDL addresses "Copying in Quantity." If the licensee distributes "printed copies" in quantities of more than a hundred copies, the licensee must abide by all of the requirements of Section 2 already described and must also abide by the provisions of Section 3. These provisions require the licensee to put the Front-Cover Texts on the front and the Back-Cover Texts on the back of copies. Section 3 also requires that the title of the Document be placed on the front cover of such printed copies. Such copies must also identify the licensee as the publisher of these copies on the front cover. Note that the licensor (usually the author of the Document) doesn't have to require that Cover Texts be distributed--in which case these additional requirements do not apply.

Section 3 also requires that if the copies distributed in quantity are Opaque, as defined above, Transparent copies of the Document must accompany each Opaque copy or be readily available to the public over a computer network. Transparent copies serve the same purpose in the documentation context that open source code does in the software context; i.e., making the creation and distribution of modified versions of the Document as easy as possible. While it would not be impossible to create a Transparent version of an Opaque copy of a document (for example, by scanning printed pages), the FDL encourages modification by putting the relatively minimal burden of making Transparent copies on the distributors of the licensed Documents, not the recipients. Obviously, the author or copyright holder is almost always in a better position to create a Transparent copy of a Document than the recipient of such a Document, because the author almost certainly created the Document in an electronic format with a word processor or text editor.

Section 4 addresses modifications to the original Document and is by far the most complex section of the FDL. However, almost all of the requirements of this section are structured around the two principles: first, that the modified Document be clearly identified as being distinct from the original Document, and second, that the original Document be clearly identified as a predecessor to the modified Document.

In order to keep the original and modified Documents distinct, the distributor of the modified version must:

  1. give a distinct title to the modified Document
  2. identify the authors of the modified Document
  3. state the name of the publisher of the modified Document on the cover page
  4. add an appropriate copyright notice for the modifications
  5. include a copy of the FDL and indicate that it applies to the modified Document.

The modified Document can't carry any endorsements that may have applied to the original Document--which makes sense, because anyone who may have endorsed the original Document endorsed a work different, however much or however little, than the modified Document.

In order to reflect the fact that the modified Document is derived from the original Document, the "Modified Version" must:

  1. contain a section entitled History (or add to one if there is one already in the original Document) listing the title, authors, and
  2. publisher of the original Document (and any of its predecessor Documents), as well as those of the modified Document
  3. list at least five of the authors of the original Document on the title page
  4. preserve and reprint all of the copyright notices applicable to the original Document
  5. preserve the Cover Texts and Invariant Sections, if any, from the original Document
  6. preserve the reference to the publicly available network location of the Transparent version of the original Document, if there is such
  7. preserve any Acknowledgments or Dedications section from the original Document in the modified Document.

While this list of conditions may seem quite daunting, for many documents, the task should be relatively straightforward. If there aren't any Invariant Sections, Endorsements, Acknowledgements, Dedications, or Cover Texts, the creator of the modified Document needs only to change the title, include a history section, list the author(s) of the original Document along with his or her own name, and, of course, license the modified Document under the FDL.

Much of the complexity of the FDL is due to its treatment of these "additional" sections--the Invariant Sections, Endorsements, Acknowledgements, Dedications, or Cover Texts--many (or all) of which are probably unnecessary to the real purposes of the Document. Authors who are considering using the FDL may want to forego the inclusion of this section in order to simplify others' compliance with the terms of the FDL and to encourage the free modification of their Document.

Nonetheless, the FDL permits the author of the modified Document to add his or her own versions of these sections--meaning that a modified Document, after several generations, could have a number of Front Cover and Back Cover Texts, as well as an extensive list of Invariant Sections, Acknowledgements and Dedications, all of which must be included in distributions "in quantity," and all of which, excepting Front Cover and Back Cover Texts, must be included in any distribution of the Document. This may, at some point, lead to undesirable results--it certainly could be possible, although unlikely, that such "surplus" sections could grow to be longer than the text of the "useful" part of the Document.

Distribution of a modified Document, once the conditions of Section 4 are satisfied, is exactly the same as for an original Document under either Section 2 or 3, depending on the circumstances (i.e., whether or not the distributor is distributing more than one hundred printed copies). Finally, Section 4 prohibits the use of the name of the author or the publisher of original Document for use in publicity or as an implied endorsement of the modified Document.

For a variety of reasons, authors may not want to encourage free modification of their works. Such authors should not use the FDL, but should use either one of the Creative Commons licenses or a variant on the Open Publication License.

The remainder of the FDL mainly addresses housecleaning and situations in which space can be saved by reprinting only one version of a particular text.

Section 5 address "Combining Documents." When two (or more) documents are both released under the FDL, the resulting Documents need have only one copy of the FDL itself attached and need not reprint more than one copy of identical Invariant Sections, such as, for example, if the Documents being combined were written by the same author who included the same philosophical explanation for his choice of the FDL with both Documents. Every other aspect of the FDL must be complied with in connection with both of the Documents: all Acknowledgements, Dedications, Cover Texts (if applicable), and non-identical Invariant Sections must be reproduced. The history section and list of authors must reflect both source Documents. Nota bene: Documents may be combined in this way only if both Documents are licensed under the FDL.

Section 6 applies to collections of Documents that are all licensed under the FDL. In such circumstances, the distribution of all the Documents need only have one copy of the FDL license text, but otherwise must comply with all of the license requirements with regard to each individual Document, with regard to Acknowledgements, Dedications, Invariant Sections, and so forth.

If the distributor wishes to reprint an FDL-licensed Document along with texts under other licenses, he or she may do so. Section 7 explicitly permits such a distribution so long as all of the requirements of the FDL are complied with regarding the FDL-licensed Document. Obviously, such a distribution must also comply with whatever other requirements are imposed by the licenses applicable to the other texts. The FDL will not apply to those other texts. A similar provision of the GPL permits copies of GPL- licensed programs to be distributed on the same media as non-GPL licensed programs, without violating the GPL. The distribution of works on the same media, however, is completely distinct from the combination of FDL and non-FDL licensed works (and GPL and non-GPL licensed works) which is not permitted by the FDL (or the GPL).

Section 8 addresses translations of Documents. This section states that translations are Modified Versions of the Document that must comply with all the requirements of Section 4. While the licensee producing such a translation may wish to translate Invariant Sections of the Document or the FDL itself, among other such texts, he or she may do so, but need not. The original, untranslated form of these texts, however, must be reproduced.

Section 9 addresses termination; that is to say, the effect of ending the relationship created by the license. This provision parallels a nearly identical section in the GPL. Any use, modification, or distribution of the Document other than as permitted by the FDL results in a termination of the putative licensee's rights under the FDL. If, however, such a licensee had distributed copies of the FDL-licensed documents to others prior to violating the FDL, those persons' rights remain unaffected by the breach.

The Free Software Foundation, the author of both the GPL and FDL licenses, does provide a list of licenses that it considers to be compatible with the GPL. Works under such licenses can be combined with GPL-licensed works without violating the GPL, at least in the eyes of the Free Software Foundation.

Section 10 simply states that the Free Software Foundation may issue different versions of the FDL that will have distinct sequential numbers. In the event that a licensee receives a work that is identified as being licensed under the FDL but that does not provide a version number, the licensee can choose any version of the FDL to apply to his or her use of the work.

Overall, the FDL provides a functional and detailed framework for the application of the license principles articulated in the GPL to documentation. For the most part, licensees have a clear idea of their rights and corresponding obligations under the FDL. The FDL guarantees that works licensed under it will be readily available for modification and distribution.

Nonetheless, there are some problems with the FDL, some the result of its philosophy and some as a result of drafting choices. In terms of its philosophy, the FDL creates complications by permitting authors to include Invariant Sections whose sole purpose seems to be to describe the relationship between the work's authors and the work. While the goal is a laudable one, it makes the license substantially more complicated, and perhaps worse, may swell the size of Documents over time because Invariant Sections cannot be changed or omitted.

In addition, the FDL has some ambiguities that could have been cleared up by slightly better drafting. The FDL contains a number of terms that are poorly defined: it's not clear, for example, whether Acknowledgements or Endorsements (or the Title Page, for that matter) are Invariant Sections, part of the Document, or Secondary Sections that need not be included. It seems more likely than not that they are intended to be either Invariant Sections or part of the Document (as the FDL seems to make clear that all of these elements must be contained in any copy or modification of the Document), but it would certainly clarify matters if there was language in the FDL to that effect. This a relatively minor failing, however.

Overall, the FDL provides a durable, relatively comprehensible framework for the licensing of documents. While it has not experienced anything near the success of the GPL, this likely was not intended. The FDL is more a companion to the GPL than a freestanding license. While numerous manuals and other technical documents have been licensed under the FDL, it is an open question whether the FDL will succeed outside of this relatively narrowly confined niche of materials that supplement GPL or other open source-licensed works.

In part two in this three-part series, Andrew will cover the Open Publication License. Stay tuned.

Andrew M. St. Laurent is an experienced lawyer with a long-time interest in intellectual property, particularly software licensing.


In August 2004, O'Reilly Media, Inc., released Understanding Open Source and Free Software Licensing.


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