The St. Louis class light cruisers were a pair of warships that served in the US Navy during World War II. The class was a slight modification of the seven-ship Brooklyn class that immediately preceded them, incorporating new higher pressure boilers and a new boiler arrangement, with machinery on the "unit system": alternating boiler and engine rooms to prevent a ship from being immobilized by a single unlucky hit. Additionally, AA armament was improved. They were the first US cruisers to be armed with twin five-inch (127 mm) 38-caliber guns. They could be distinguished visually from the Brooklyns by the placement of the after deckhouse, immediately abaft the second funnel, and by the twin 5" mounts.