The Nevada-class battleships were the United States Navy's first battleship design equipped with triple gun turrets (the Colorado class would be the last to carry twin turrets, armed with dual-mounted 16-inch guns), as well as introducing the "all or nothing" armor scheme in American capital ship design, in which protection of vital areas was optimized against heavy caliber guns, leaving other parts of the ship essentially unprotected. The Nevadas also introduced completely oil-fired propulsion. In armament, armor, and propulsion the Nevada class represented a considerable evolution in battleship design and, in being designed specifically to fight at extreme gunnery ranges, was actually well ahead of its time. They would be followed by the Pennsylvania-class battleships.