Whats the point of dual-licensing?

Question

I am wondering what the point of dual licensing is whenever the two licenses in question are both open source licenses.

For instance, jQuery provides their library under either the MIT or GPL license. What is the point of doing this?

Also, I've seen http://stackoverflow.com/questions/1174271/what-happens-when-open-source-software-has-two-or-more-licenses as a possible duplicate, but they don't answer why jQuery is actually licensed this way.

Answer 1

It's slightly uneccessary - more a political statement of support for the GPL.
Or to make it obvious that you can include it in the GPL and so stop managers/lawyers worrying

MIT (or rather X11) licenced software is automatically GPL compatible,

Answer 2

Including code under different licenses in a project can really be a problem. A larger project could contain code under many different licenses. So offering multiple licenses can help the user to clean up this jungle.

Answer 3

Dual licensing is usually about removing as many licensing barriers from using your software as possible.

Roughly the free software world splits into GPL and BSD style. GPL being about viral freedom, BSD being about totally free use. Dual licensing your software both GPL and BSD eliminates licensing from consideration of use. You just use whichever one fits your style. This can stop licensing wars before they start. There is no question of whether your project is GPL compatible, you declare it up front.

The advantage of dual licensing comes out when you start bundling software together. Can GPL'd software be bundled in a BSD licensed project? What about vice-versa? These are difficult questions and dual licensing makes them go away.

Dual licensing a project as both GPL and BSD defangs the GPL. Its viral freedom clauses are effectively unenforceable because the user can simply use it under the BSD terms. Depending on which side of the Free Software fence you sit this is a good or bad thing.

Finally, dual licensing causes some issues when pulling in code from other projects. You must make sure that the code you're copying qualifies for both licenses. Generally when copying code it has to qualify for the more permissive of the two licenses. JQuery cannot pull in GPL licensed code, or they'd be violating the GPL by offering GPL code under the more permissive MIT license. They can pull in MIT licensed code, since you can redistribute MIT licensed code under the GPL. They may not be able to pull in new BSD licensed code since the MIT license does not have a "no endorsement" clause, and is thus more permissive, but likely nobody really cares about that clause.

That said, I Am Not A Lawyer.

Answer 4

With a GPL licensed version:

1. Open Source advocates and developers or products are happy to include it.
2. You can accept contributions to the product by outsiders
3. You can argue the product has the traits of Open Source software, e.g. bug fixes and free world wide security white box testing

With a closed source license:

1. Sell it to companies at a profit for money. In exchange they can make code changes without having to "open source" them when the redistribute the product.

Dual licensing gives you the best of both worlds.

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One final issue is that your product can be mixed with similar licenses. For example GPL source code can only be statically linked to other GPL binaries I believe. Multiple licenses enables greater binding of different pieces of software.