This morning a short question was posed on one of the Creative Commons mailing lists (cc-community).

I have a simple question. Why are all the Creative Commons licenses incompatible with the GPL?

This was an excellent little question and since then the mailing list has been busy sending in responses and thoughts. Since this is an open mailing list it is ok to quote one of the answers which I found very well written and helpful in explaining this important issue. The reply comes from Greg London and is as follows:

(broad brushstrokes follow.
Nit-pickers need not apply)

If you’re talking about converting
content between the CC-SA and GNU-GPL
licenses, then the problem is basically
a side effect of copyleft.

Copyleft licenses keep the content Free
by demanding that the content and any
derivatives are always available under
the same license as the original.

This prevents someone from putting more
restrictions on the work and taking a
version of Free content private.

Almost counter intuitively, copyleft
protects the content by disallowing
someone from removing restrictions on
the work. This could be abused by allowing
someone to first convert the content from
a copyleft license to a public domain license,
and then allowing the person to create
proprietary forks.

So, copyleft keeps the work Free by demanding
that the content and its derivatives must always
be held under the same license as the original.

Which means that if you have two copyleft
licenses, but they have different requirements,
they are incompatible. The GNU-GPL and CC-SA
licenses are both copyleft. But the GNU-GPL has
a source code requirement that the CC-SA does not.

If you took CC-SA content and converted it to
GNU-GPL, you would be adding a source code
requirement to the content that did not exist
before. And if you took GNU-GPL content and
converted it to CC-SA, you’d be removing the
source code requirement.

And since both say you can’t change the requirements,
converting between either license is disallowed.

The idea CC is apparently working on for making
licenses inter-operable is to put language into
the license that allows the content to be licensed
under the original license, or any license that is
deemed to be similar enough, for some fuzzy definition
of “enough”.

They already have something like this that makes sure
that, for example, the different language versions
of CC-SA are compatible with each other. The way I
understand it, they’re are going to try to use the
same approach to expand compatibility outside of the
CC-SA licenses.

Since no CC license has a “source code” requirement,
I don’t think any CC license will ever be directly
interchangable with GNU-GPL. But they are trying to
solve the problem of license proliferation by building
in a mechanism that will allow all the content to be
transferred to licenses that are deemed “close enough”.

I hope this helps.

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