Joomla! Documentation™

Major Release Number (X.Y.Z) - An increment of the major number 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 code base, parts of Joomla! 1.0.x still reside in the code base
• 2.5.x - major rewrite of the code base moved to a largely Object Oriented code base from 1.5
• 3.x.x - more minor update of the code base - large change in template functionality to use bootstrap 2
• 4.x.x - cleanup of code base to remove deprecated features - but still recognisably like Joomla 3, upgrade from bootstrap 2 to 5

A full description of the Version Strategy and the release versioning is available.