The "Illustrious" class comprised four vessels: HM Ships "Illustrious", "Formidable", "Victorious" and "Indomitable". The last of these was built to a modified design with a second, half-length, hangar deck below the main hangar deck. Each of these ships played a prominent part in the battles of the Second World War. "Victorious" took part in the pursuit of the German battleship "Bismarck", "Illustrious" and "Formidable" played prominent parts in the battles in the Mediterranean during 1940 and 1941 and all three took part in the large actions of the British Pacific Fleet in 1945.