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Part of this license outlines requirements for derivative works, such as plugins or themes. Derivatives of WordPress code inherit the GPL license. Drupal, which has the same GPL license as WordPress, has an excellent page on licensing as it applies to themes and modules (their word for plugins). |
There is some legal grey area regarding what is considered a derivative work, but we feel strongly that plugins and themes are derivative work and thus inherit the GPL license. If you disagree, you might want to consider a non-GPL platform such as Serendipity (BSD license) or Habari (Apache license) instead.
Licensing FAQ | drupal.org
9: Is it permitted for me to sell Drupal or a Drupal module or a Drupal theme? |
Yes. However, you must distribute it under the GPL version 2 or later, so those you sell it to must be allowed to modify and redistribute it as well. See questions 6 and 7 above.
7: If I write a module or theme, do I have to license it under the GPL? |
Yes. Drupal modules and themes are a derivative work of Drupal. If you distribute them, you must do so under the terms of the GPL version 2 or later. You are not required to distribute them at all, however. (See question 8 below.)
When distributing your own module or theme, therefore, the GPL applies to any pieces that directly interact with parts of Drupal that are under the GPL. Images and Flash files you create yourself are not affected. However, if you make a new image based off of an image that is provided by Drupal under the GPL, then that image must also be under the GPL.
If you commit that module or theme to a Drupal Git repository, however, then all parts of it must be under the GPL version 2 or later, and you must provide the source code. That means the editable form of all files, as described above.