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Joomla! is a fork of Mambo, or the other way around? According to this article in the Joomla! Developer Blog, in August 2005 a dispute developed within the Mambo Steering Committee. “The entire Core Team decided to sever ties with Miro, the company supporting Mambo, and leave the project. They regrouped under another name and released Joomla! 1.0 in September 2005.

Another way to look at it is like this:

Mambo today: same name, same base code, different team;
Joomla! today: different name, same base code, same team.

This makes it hard to answer the question “what is the original”. Joomla! 1.0 and Mambo 4.5.3 should be mostly compatible, being both based upon Mambo 4.5.2. Following the release of Joomla! 1.5 and Mambo 4.6 we are likely to see polarisation of the two projects and each provide its own richness and new flavours to the Open Source movement.”