I was interested in the recent conversation about whether WordPress themes need to be licenced under the GPL or whether they can be closed source. The video can be reached from matts blog and my favourite discussion about it is Dave Winers blog post.
My question is – am I allowed to license my theme under the aGPL? or is this the same argument as if I wanted to use a closed source license?
Since WordPress is licensed as GPLv2, you can license any addition (i.e. theme) with a license that’s compatible with version 2 of the GPL. Now here’s the unfortunate part … version 1 of the aGPL is not compatible with any version of the GPL. Version 3, however, is compatible with GPLv3 … but not GPLv2.
Short answer: no, you can’t license a WordPress theme with aGPL because neither version of the aGPL is compatible with WordPress’ license (GPLv2).
Link to license compatibility chart for reference …
Strictly speaking, yes. WordPress themes are considered derivative works, so they must be licensed in a fashion compatible with the core project. WP is licensed as GPLv2+, so you can (if you want to) upgrade it to GPLv3 for distribution.
That’s important, because the only version of the aGPL that’s compatible with the GPL is version 3 … so your theme must be aGPLv3.
That said, you’re under no expectation to redistribute WordPress on your own … I’m just pointing out the compatibility here. But to keep things simple, I will always advise against using the aGPL. In practice, it doesn’t lend well for WordPress themes (feel free to ask me why, but that’s beyond the scope of this question).
One sentence summary from Matt Mullenweg : PHP in WordPress themes must be GPL, artwork and CSS may be but are not required.
Adding a reference to updated @EAMann’s answer: I’ve asked similar question to firstname.lastname@example.org.
My exact question was:
Could you please clarify what does it mean to release source code under “GPLv2 (or later)” and if so, if it allows to release derivate works under aGPL v3.
And the answer that came:
By licensing their work under GPLv2 (or later), which I will refer to in
this email as GPLv2+, the copyright holders of WordPress have explicitly
permitted you to further distribute their work under any later version
of the GPL. By upgrading you can further distribute their work under
GPLv3, GPLv3+, GPLv4 (doesn’t exist yet, but if it did you could choose
it), GPLv4+, etc. This is spelled out in section 9 of GPLv2 and section
14 of GPLv3.
Once you’ve upgraded the work to GPLv3 (you update the license version
number and include a copy of GPLv3, add your own copyright notices as
needed, but otherwise keep the original copyright notices and license
notices intact), two things will happen: First, the work becomes
incompatible with GPLv2-only code; both GPLv2 and GPLv3 are strong
copyleft licenses and they cannot both be satisfied at the same
time. Second, and more important for you is that under section 13 of
GPLv3, and section 13 of AGPLv3, you will now have a narrow
compatibility with works licensed under AGPLv3. Which is to say that
while you cannot re-license the work, as a whole, from GPLv3 to AGPLv3,
you will be able to further distribute the combination of GPLv3 code
(the upgraded WordPress code) and AGPLv3 code (your additional code).
volunteer at the FSF GPL Compliance Lab
Yes, you are allowed to license your theme under any license you see fit. Some might not be appropriate for redistribution then. But first of all you are free to choose. It’s free software, extend as you wish.