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. Some of the categories below are not in U.S. naval service, but are in active service with other navies.

Codes have from two to four letters. For vessels in current use, the first letter will be:

• A: Auxiliary; formerly included amphibious warfare
• C: Cruisers and aircraft carriers; some ships that may appear to be aircraft carriers are, due to their mission, considered amphibious warfare types. Some NATO navies use an R prefix for aircraft carriers
• D: Destroyer
• F: Ocean escorts, called frigates in the current United States Navy but having an extremely wide range of designations
• L: Amphibious warfare
• M: Mine warfare
• P: Fast attack craft, possibly other vessels for littoral warfare
• S: Submarine
• W: United States Coast Guard vessel
• Y: support craft, often inshore such as tugs and dredges

A T-prefix, usually to an auxiliary type, denotes, a ship crewed by civilian mariners but in service of naval forces.

Additional letters, in a complex and sometimes inconsistent manner, further qualify the first letter.

Ship names

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.

The warship versus auxiliary issue is primarily a U.S. and U.K. issues. Not all navies even use a prefix for names.

Many do assign a number following the full type code; at various times, different navies have not necessarily given a name to a ship type. Many submarines, prior to or during the Second World War, simply had numbers, such as S-10 (pre-WWII U.S.), U-234 (German), I-151 (Japanese).