From Joomla! Documentation
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Major Release Number (X.Y.Z) - An increment of the major number generally indicates a major rework or rewrite of the code base (framework level). Is most likely incompatible with prior major releases.
Currently Joomla! has released the following major releases:
- 1.0.x - code base derived from Mambo 4.5.3
- 1.5.x - major rewrite of the codebase, parts of Joomla! 1.0.x still reside in the codebase