Background: December 2009, New York City When the newly-formed Production Leadership Team (PLT) met in New York City in December 2009, we faced a challenge. Although Joomla! version 1.5 was popular and successful, it had been released almost two years earlier. The code for version 1.6 contained many incomplete and untested features, and no one could say how soon version 1.6 might be ready for release. (It turned out to be a little over a year, in January 2010.)...
Read more http://magazine.joomla.org/issues/Issue-Apr-2012/item/716-Joomla-Versions-and-Updates-Explained