license-mix, legal consequences?

[John Cowan]

Henningsen wrote:

> My understanding is this: As the copyrightholder to my code, even if I
> release it under the GPL, I am not bound by the terms of the GPL, and can
> include code under a different license such as the MIT license. However, if
> someone else uses my code under the terms of the GPL, and wants to release a
> modified version of my code, then according to 2.b) he would have to license
> the entire program under the terms of the GPL, *including the parts that he
> received under the terms of the MIT license*. I had been thinking that it
> would not be legal to simply take MIT licensed code (or Tcl/Tk licensed
> code) and without permission of the copyright holder re-release it under the
> GPL.


In order to understand questions such as this, one must maintain a vital
distinction in mind:  the difference between the license applied to
an *original* work, and the license that may be applied to a
*derived* work such as a program made using sources from various
authors.  In either case, it is the creator of the work who decides
what licenses to apply, if any.

1. You may license your original works under any license(s) you choose.

2. If someone else assigns you copyright in their works (and this must
be done in writing), then you may license those works under any
license(s) you choose.

3. If you make a derivative work containing others' content, you may
license it under any license(s) that are compatible with the licenses
applicable to each of the original works that you used.  You may not
change the licenses applied to those original works.

Suppose there are three original works:  A under the GPL, B under the
MIT, C under the MPL.  You may create a derivative work A+B and license
it under the GPL.  You may not license A+B under the MIT, because
you are not allowed to create derivatives of A that are not licensed
under the GPL.

You may not create a derivative work A+C at all, because
the MPL imposes restrictions that are incompatible with the GPL:
A+C cannot be licensed under either the GPL or any other license.
But if A belongs to you (in the sense of points 1 or 2 above), then you
may license it under the MPL as well as the GPL, and then A+C can be
created and licensed under the MPL.  If C does not belong to you,
then you cannot relicense it under the GPL.

-- 
There is / one art             || John Cowan <jcowan at reutershealth.com>
no more / no less              || http://www.reutershealth.com
to do / all things             || http://www.ccil.org/~cowan
with art- / lessness           \\ -- Piet Hein