From 1506 to 1626, it changed from the simple ground plan of a Greek crose with four arms of equal length, as favored by Bramante and his arch-enemy Michelangelo, to the final form of Maderno's Latin croseextended by a long nave, as demanded by the popes of the Counter-Reformation.
After the end of the Counter-Reformation, some people wanted the design to be changed back.