For more than a hundred years, U.S. Navy ships were known only by type and name, as in “the frigate Constitution.” But as more and more ships entered service, this practice became cumbersome and often confusing, as ship names sometimes were repeated—by 1874 there had been five ships named Enterprise (with four more to come, not counting the starship!).
It was becoming clear that a more distinguishing system was needed, and in the 1890s the Navy began assigning unique “naval registry identification numbers” to its ships. This simple system called for the name of the ship followed by its type (spelled out) and a unique number for the individual ship of that type. For example, the first cruiser was the Newark (Cruiser No. 1), the second the Charleston (Cruiser No. 2), and so on. In 1907 this system was modified to allow abbreviations, using just a “C” for cruisers, as in the Olympia (C-6), a “B” for battleships, as in the Massachusetts (BB-2), “ACR” for armored cruisers, as in the Maine (ACR-1), and so on.