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If I use a compiled program/non compiled source code which is under a license that everyone has the rights do use and sell (Eg: GPL) and no one can deny any person to use or sell it and then I want to develop an extended feature for it/document/presentation/compiled version for it to be sold for money, to be available only to the person that payed for this premium feature - Must I release this premium feature also as GPL ? If I must than it means that the person that just payed money for this premium feature can distribute it or sell it too or do whatever he want without paying me further.

If someone develop a premium feature - how can he protect his premium feature for further free distributions that will eventually stop him of getting pay for his premium work. If this premium feature will be available to all - then why would someone want to continue paying for it ?

For example: If I develop a wordpress plugin. (Wordpress is GPL). I develope a free plugin under the GPL too which everyone can take and do what they want with it (Almost), then I develop a better version of this plugin for money. Must it also be GPL ? If it does that as I wrote above - everyone can do whatever they want with it and I won't have the ability to continue selling it for a long time.

What is the solution for that ? Thanks !

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If I use a compiled program [...] which is under a license that everyone has the rights to use and sell (Eg: GPL) [...] and then I want to develop an extended feature for it [...] to be sold for money, to be available only to the person that paid for this premium feature - Must I release this premium feature also as GPL?

If your work is a derivative of the original GPL work, then yes, you must. GPLv3 requires this in s5c ("You must license the entire work, as a whole, under this License to anyone who comes into possession of a copy"). Older GPL versions, and other copyleft licences have comparable requirements. If the original work was licensed under a permissive free licence, then no, you may not have to do this.

Whether your work is a derivative of the original is a complex question, but from what (little) you've told us, I for one suspect that it is.

it means that the person that just paid money for this premium feature can distribute it or sell it too or do whatever he want without paying me further?

Yes, that is exactly what it means. It further means that any of those people to whom your client distributes it also get similar rights under the GPL.

If someone develops a premium feature - how can he protect his premium feature [against] further free distributions that will eventually stop him getting pay for his premium work.

He can't. That is the whole point of the GPL - to ensure that all users who get the program get freedom along with it. The author of the original work intended that this should be the case, which is why (s)he picked the GPL in the first place. If you don't like that, don't work on GPL code; or better, price your work so that you're paid fairly for it by the person who gets the first copy, and so have no need to make more money by shaking down other users.

If this premium feature will be available to all - then why would someone want to continue paying for it ?

Because they want that feature? I have been involved in paying money to free software developers in order to prioritise features that I had a particular need for. I was very happy to get those features added, and it in no way diminished me that others had access to them also.

The specific question about Wordpress plugin licensing is addressed in several places on this site already, eg here and here.

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    It's an excellent answer which covers some aspects not mentioned in the already very good answer to the question I marked this one a duplicate of – planetmaker Mar 20 at 5:14
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If it is documentation, you can e.g. write a book on emacs, or GNU MP (one example among many), Python or Perl, keep it's source closed and sell it. This is quite common, just take a peek at O'Reilly's offers, some are open source, but not all. Note the software described is under different licenses.

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