Why WordPress Themes are Derivative of WordPress

“There are many aspects to the ongoing WordPress vs. Thesis
brouhaha. Things like respect, copyright, freedom, GPL, ownership.
There is no doubt that Thesis lifted lines of code, and even whole
sections of code, directly from WordPress. The Thesis developer who
did some of the lifting confessed to it (much to his credit).

“Some people have changed their opinion on the matter when they
learned of that wholesale code copying. That is, they changed their
mind about Thesis specifically. But those revelations happened
later in the fray, after Chris Pearson and Matt Mullenweg’s
discussion on Mixergy about the matter. It is the position of the
WordPress core developers that themes cannot be considered wholly
original creations even when they don’t copy large sections of code
in from WordPress. Theme code necessarily derives from WordPress
and thus must be licensed under the GPL if it is distributed.”

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