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WordPress is licensed under (GPLv2 or later). WordPress states that all plugins hosted under the WordPress Plugin Directory must be licensed under GPL.

We have a Free version of our plugin that is licensed under GPL and published in the WordPress Plugin Directory.

We also have a Pro version of our plugin that we offer to our Free users, that adds extra functionality and features. To get the Pro version, the user has to create an account on our website, pay and download a .zip file of the Pro version of the plugin.

According to GNU, as the copyright holder I can release the same software under two different licenses.

I don't consider our plugin to be a derivative of WordPress, because it was built from scratch: We didn't fork anything from WordPress and built a new version of it - it's brand new work.

Does the Pro version of my plugin that the user downloaded from my website must be licensed under GPL as well? Can I license it under a proprietary license?

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You have to use the GNU GPL version 2 or later.

A theme or plugin is a derivative work of WordPress because it uses WordPress functions, hooks, etc.

But this doesn't prevent you from selling your plugin! You are free to require a payment to get the plugin, but it has to be under the GNU GPL version 2 or later.

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I'll answer my own questions based on my own investigation.

GPL2 states:

If identifiable sections of that work are not derived from the Program, 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.

From my understanding, a plugin that I write myself and I sell as a standalone deliverable is a separate work, thus not bound to GPL2.

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  • You have to use the GNU GPL version 2 or later. A theme or plugin is a derivative work of WordPress because it uses WordPress functions, hooks, etc. But this doesn't prevent you from selling your plugin! You are free to require a payment to get the plugin, but it has to be under the GNU GPL version 2 or later.
    – JNic
    Dec 15 '20 at 13:31
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    The problem with that reasoning is, that by most accounts, a plug-in is not considered an "independent and separate work": From the FAQ: "If the main program dynamically links plug-ins, and they make function calls to each other and share data structures, we believe they form a single combined program, which must be treated as an extension of both the main program and the plug-ins. If the main program dynamically links plug-ins, but the communication between them is limited to invoking the ‘main’ function of the plug-in with some options and waiting for it to return, that is a borderline case" Dec 15 '20 at 18:12
  • @planetmaker Considering that a derivative and a separate work is subject to interpretation, let's say WordPress officially considers Premium being a separate work, thus not bound to GPL2. If WordPress, as the copyright holder of the original project, assumes this instance, can someone say otherwise? Dec 16 '20 at 12:53
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    Many people can say many things ;) It probably depends on jurisdiction, but here in Europe only the copyright holder can sue for infringement. Dec 16 '20 at 15:18
  • @JNic I believe this to be a matter of interpretation by the copyright holder. In this case, WordPress. More specifically, Matt Mullenweg. For instance, Linux is GPL, but Linus Torvalds, the copyright holder, has decreed that proprietary kernel modules are allowed to have proprietary licenses, even though they interface with Linux intimately. Dec 18 '20 at 17:07

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