An armoured flight deck is an aircraft carrier flight deck that incorporates substantial armour in its design. Comparison is often made between some of designs of the Royal Navy (RN) and the United States Navy (USN). The two navies followed differing philosophies in the use of armour on carrier flight decks starting with the design of the RN's Illustrious class and ending with the design of the Midway class, when the USN also adopted armoured flight decks. The two classes most easily compared are the RN's Illustrious class and Implacable class and their nearest USN contemporaries, the Yorktown and Essex classes. The Illustrious class followed the Yorktown but preceded the Essex, while the Implacable class design predated the Essex but these ships were completed after the lead ships of the