Why carrier numbers determine availability

The availability ladder begins with one carrier, which can disappear into refit, training, or the wrong geographical position. Two add utility, three improve the chance of having a ready ship, and four provide a more reliable capability for one problem at a time. The required number still depends on a state's commitments and the number of theatres it may have to cover. Britain once combined three Invincible-class ships with HMS Ocean, giving it four large flight decks even though Ocean could not operate Harriers. Carriers are combat platforms and highly visible conventional deterrents whose credibility depends on availability.