The "Illustrious" class was designed within the restrictions of the Second London Naval Treaty, which limited carrier size to an upper limit of 23,000 tons. They were different in conception to the Royal Navy's only modern carrier at the time, their predecessor HMS "Ark Royal", and what may be described as their nearest American contemporaries, the "Yorktown" and "Essex" class carriers. The "Illustrious" class followed the "Yorktown" but preceded the "Essex", the latter being designed after the US abandonment of the Second London Naval Treaty and its tonnage limitations.