What is Copyleft?

Frank Hecker frank at collab.net
Thu Feb 22 19:37:47 UTC 2001

"Ryan S. Dancey" wrote:
> Here's a question I thought I'd never have to ask.
> What is a Copyleft?
> The reason I ask this question relates to RMS's recent pronouncements about
> Apple's psuedo-open license terms.  He says, in part, that one of the flaws
> of the license is that:
> "It is not a true copyleft, because it allows linking with other files which
> may be entirely proprietary."

For what it's worth, the FSF discussion of "Various Licenses and
Comments about Them"


makes a distinction between licenses that are "copyleft" licenses,
licenses that are "not a strong copyleft", and "non-copyleft" licenses.
Thus the GPL is described as a "copyleft license", but the LGPL is
described as "not a strong copyleft license, because it permits linking
with non-free modules"; the MPL (which permits both static and dynamic
linking with proprietary code) is also described as "not a strong
copyleft". On the other hand the MIT X license and Apache licenses are
described as "non-copyleft".

So I think the issue at hand is not that the meaning of copyleft is
directly tied to the issue of linking, it's that the FSF uses the issue
of linking to try and introduce a concept of the relative "strength" of
copyleft as embodied in different license.

Frank Hecker            work: http://www.collab.net/
frank at collab.net        home: http://www.hecker.org/

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