Destroyer escort (DE) was the United States Navy mid-20th century classification for a 20-knot (23 mph) warship designed with endurance to escort mid-ocean convoys of merchant marine ships. Kaibōkan were designed for a similar role in the Imperial Japanese Navy. The Royal Navy and Commonwealth forces identified such warships as frigates, and that classification was widely accepted when the United States redesignated destroyer escorts as frigates (FF) in 1975. Destroyer escorts, frigates and kaibōkan were mass-produced for World War II as a less expensive anti-submarine warfare alternative to fleet destroyers.