WordPress and the GPL
Open Source & Derivative Works
WordPress & the GPL
ProCD v. Zeidenberg
USL v. BSDi
Shrink Wrap License
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MS Permissive License
WordPress and the GPL
WordPress is blogging software that is licensed under the GPL v2 or later. The WordPress software provides provides the basic platform for blogging, but in order to create more attractive looking blogs, or to add features, users can extend the software using Themes (for appearance) or Plug-ins (to add features). A huge variety of Themes and Plug-Ins for WordPress have been developed for WordPress, and many are show cased on the WordPress website.
One of the requirements of the GPL is that third parties may distribute the software, but if they do so, they must distribute the software under the GPL. Furthermore, derivative works of GPL software, must themselves be distributed under the GPL.
According to the creators of WordPress, Themes and Plug-Ins are generally derivative works of WordPress, and therefore if they are distributed, they must be distributed under the GPL: Distributing under another incompatible license is GPL Violation (and therefore a form of copyright infringement).
To bolster their position, sought a letter from the Software Freedom and Law Center (SFLC). Their conclusion, expressed in a published letter, was that WordPress Themes are generally derivative works of WordPress, therefore are subject to the GPL. They did however state the CSS and image files in a Theme WordPress are generally not derivative works and therefore do not have to be licensed under the GPL. Additionally, the SFLC did also state that it might be possible for someone to develop WordPress Themes which are not derivative works, but to do so that would have to forego almost all the features and functionality that make a Theme useful.
On the other hand, some developers of commercially licensed Themes and Plug-Ins have asserted that their products are not derivative works of WordPress, and therefore do not have to be licensed under the GPL.
As far as I know, no court or judge has looked at this issue, or even a particular example of this issue (because it could even be the situation is different from different programs/Plug-Ins/Themes), so it's currently a matter of opinions. Who is right does however have important consequences: