Within the WordPress community, the question of GPL inheritance of WordPress themes erupts into contentious debate with the reliability - if not the frequency - of Old Faithful. While I understand that, according to the GPL interpretation of Matt Mullenweg, the Free Software Foundation (FSF), and the Software Freedom Law Center (SFLC), WordPress themes are derivative of WordPress and therefore must necessarily inherit WordPress' GPL, I would like to investigate the issue not in light of their interpretation but rather in light of copyright law and precedent case law.
Before I begin, let me add an important caveat: I have no qualms with the GPL. I have always released - and will continue to release - under GPL anything I develop related to WordPress. I do so because I choose to do so, as a means of making even a minor contribution to a project from which I believe I have personally benefited. I do have issues with how the GPL-inheritance question has been handled - but those issues are out-of-scope for this post.
Having (hopefully) made that point clear, let's begin!
What US Copyright Law Says
US Copyright law defines a "derivative work" as such:
A “derivative work” is a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications, which, as a whole, represent an original work of authorship, is a “derivative work”.
Note the key adjectives: recast, transformed, and adapted.
Consider also Section 102(b), which states:
In no case does copyright protection for an original work of authorship extend to any idea, procedure, process, system, method of operation, concept, principle, or discovery, regardless of the form in which it is described, explained, illustrated, or embodied in such work.
This clause establishes the boundary around copyright between copyrightable expression, and non-copyrightable ideas.
Summarizing GPL Inheritance Requirements
To summarize GPL requirements regarding license inheritance for derivative works 1:
- The GPL only applies to distribution of a (modified or unmodified) GPL-licensed work, or a derivative work. Any activity involving use, modification, or creation of derivative works that does not involve distribution is outside of the scope of the GPL.
- Distribution of a (modified or unmodified) GPL-licensed work, or a derivative work, requires that such distribution be licensed under GPL.
The GPL is what is now referred to as a "copyleft" license: a modified public-domain license that takes advantage of the exclusive rights granted by copyright law to prevent derivative works from being restrictively licensed. Since the copyright owner has exclusive right to produce and to distribute derivative works based on the copyrighted work, the GPL intends to grant unlimited usage rights (to use, study, modify, etc.) to the end-user, while forcing follow-on developers of derivative works to release those works under the same license.
It is important to understand that, because the GPL explicitly defines any activity not involving distribution to be out of the scope of the license, and since right of distribution is solely derived from copyright law, that GPL derives its legal basis from copyright law alone. This distinction separates the GPL from most other traditional software licenses, which derive their basis for usage and modification restrictions not from copyright law, but from contract law.
WordPress is released under GPL version 2.0. I'll try to summarize below the parts of the license germane to derivative works.
First, from the Preamble:
The reason we have a separate public license for some libraries is that they blur the distinction we usually make between modifying or adding to a program and simply using it. Linking a program with a library, without changing the library, is in some sense simply using the library, and is analogous to running a utility program or application program. However, in a textual and legal sense, the linked executable is a combined work, a derivative of the original library, and the ordinary General Public License treats it as such.
The precise terms and conditions for copying, distribution and modification follow. Pay close attention to the difference between a "work based on the library" and a "work that uses the library". The former contains code derived from the library, while the latter only works together with the library.
Terms and Conditions, Clause 0:
The "Library", below, refers to any such software library or work which has been distributed under these terms. A "work based on the Library" means either the Library or any derivative work under copyright law: that is to say, a work containing the Library or a portion of it, either verbatim or with modifications and/or translated straightforwardly into another language. (Hereinafter, translation is included without limitation in the term "modification".)
Terms and Conditions, Clause 2:
These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Library, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Library, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it.
Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Library.
Terms and Conditions, Clause 5:
A program that contains no derivative of any portion of the Library, but is designed to work with the Library by being compiled or linked with it, is called a "work that uses the Library". Such a work, in isolation, is not a derivative work of the Library, and therefore falls outside the scope of this License.
However, linking a "work that uses the Library" with the Library creates an executable that is a derivative of the Library (because it contains portions of the Library), rather than a "work that uses the library". The executable is therefore covered by this License. Section 6 states terms for distribution of such executables.