In December 2007, the Wikimedia Foundation passed a resolution requesting that the Free Software Foundation modify the terms of the GFDL so that it could relicense to Creative Commons Attribution-ShareAlike 3.0 (US) licence. The FSF released version 1.3 of the GFDL on 03 November 2008.
This is pretty big news. Wikipedia was founded before the Creative Commons licences were available, so they naturally chose the GFDL, which is the FSF's copyleft licence for text based works. The GFDL was designed to be used with manuals, help files and text books. It is quite an unwieldy licence that requires that each distribution be accompanied by the entire text of the licence. It also has express provision for invariant texts, areas of the work that are marked out to be immutable and thus non-free.
As a strong copyleft licence, the GFDL 1.2 required that all distributions of modified material be licensed under the GFDL. This means that works licensed under the GFDL are incompatible with all other licences. This means that text on wikipedia cannot be incorporated into works licensed under, for example, the GPL or a CC BY-SA licence. This means that it can become very difficult to make really interesting mashups. The paradigmatic example I use is that of an interactive educational game that combines a free software engine (GPL) with a set of wikipedia articles (GFDL), a tour guide from wikitravel (CC BY-SA v1), images from Flickr (CC BY-SA v2) and a soundtrack from Jamendo (CC BY-NC-SA v3). It may be technically possible to separate out all of the individual components (or seek extra permissions from all the copyright owners), but if it's not, the result will be a free, innovative educational interactive tourist guide which can never legally be distributed.
This is an unfortunate example of unintended incompatibility. We know that copyleft material cannot be mixed with material which is not available for free licensing, and this is desirable, because it builds the commons by encouraging people to release their modified works under a free licence. It also restricts exploitative behaviour by preventing people from taking material from the commons and locking it up for their own gain without contributing back. This is intended incompatibility. The problem comes when we make several different licences with roughly the same goals, but we draft them in such a way that they remain incompatible with each other. Because licensors rarely choose a licence based upon the particular legal wording of the licence, but instead rely upon the high level abstractions or the stature of the organisation behind the licence, then where those high level abstractions are very similar, it makes little sense for the licences to be incompatible.
The danger is that the commons we are building may still be an anticommons. In a 1998 article, Michael Heller defined an anti-commons as a situation where there were too many rights to exclude to achieve socially desirable outcomes. A tragedy of the anticommons in open content licensing occurs when authors release their content in the hope that it will be reused but under licences which unintentionally limit its utility.
This occurs when some people release material under, for example, the GPL, and others release material under CC BY-SA. At a high enough level, these licences look the same – licence modified versions under a copyleft licence, attribute the previous authors, a few disclaimers of liability, and we're done. Unfortunately, due to some differences in wording and implementation, the licences are somewhat legally different, and for these reasons, they each insist that modified versions be released only under the original licence. So, someone who wants to make use of material licensed under two incompatible licences has to ask permission from each copyright owner – exactly the same position we would be in without the licences. This is the tragedy of the anticommons.
In the past few years we've seen a drop in the use of vanity licences, and authors in the free software community have generally consolidated around a relatively small number of licences. We've also seen some significant changes by the main interested organisations. The OSI has established a License Proliferation Committee, with goals to educate licensors about their choice of licence; the FSF has introduced mechanisms in GPLv3 to enable code under previously incompatible licences (particularly Apache v2) to be integrated with code under the GPLv3; and Creative Commons has introduced a concept of Compatible Licences into version 3 of its ShareAlike licences which allow relicensing under 'approved' licences (of which there are still none). Other groups like Google Code have imposed restrictions on the number of licences that they support (but later reversed their decision and expanded the list of licences available).
So this move by the FSF has been long awaited and is very welcome. Lets take a look at what's changed.