Naval vessel designation code

Many nations use an alphabetic naval vessel designation code system based on that used by the United States. There are many historical peculiarities in the system, some reflecting obsolete types of ships, some reflecting the evolution of one type from another (e.g., aircraft carriers were considered, in their first form, a subset of cruiser), and some reflect changing names for ships with certain sets of capabilities. The term "vessel" is used rather than "ship", because certain codes are assigned to true boats, defined as vessels that can be carried aboard a ship.

Codes have from two to four letters. For vessels in current use, the first letter will be:
 * A: Auxiliary
 * C: Cruisers and aircraft carriers; some ships that may appear to be aircraft carriers are, due to their mission, considered amphibious warfare types
 * D: Destroyer
 * F: Ocean escorts, called frigates in the current U.S. Navy but having an extremely wide range of designations
 * L: Amphibious warfare
 * S: Submarine
 * W: U.S. Coast Guard vessel

A T-prefix, usually to an auxiliary type, denotes, a ship crewed by civilian mariners but in service of naval forces. In U.S. usage, their names are prefixed "USNS" (U.S. Naval Service) rather than "USS" (United States Ship). British usage can get even more complex; a warship name is prefixed "HMS" (His/Her Majesty's Ship), but some "RFA" (Royal Fleet Auxiliary) would be considered warships in the U.S. Navy.