The United States Navy uses hull classification symbols to identify the types of its ships. See also pennant number, a somewhat analogous system used by the Royal Navy and some European navies.
The combination of symbol and hull number identify a modern Navy ship uniquely. A heavily modified or repurposed ship may receive a new symbol, and either retain the hull number or receive a new one. Also, the system of symbols has changed a number of times since it was introduced in 1907, so ships' symbols sometimes change without anything being done to the physical ship.
Many of these symbols are not presently in use.
Warships are designed to participate in combat operations.
Aircraft Carrier Type
All ships designed primarily for the purpose of conducting combat operations by aircraft which engage in attacks against airborne, surface, sub-surface and shore targets. The V has an lost meaning.
Surface Combatant Type
Large, heavily armed, surface ships which are designed primarily to engage enemy forces on the high seas.
All self-propelled submersible types regardless of whether employed as combatant, auxiliary, or research and development vehicles which have at least a residual combat capability.
Patrol Combatant Type
Combatants whose mission may extend beyond coastal duties and whose characteristics include adequate endurance and sea keeping providing a capability for operations exceeding 48 hours on the high seas without support.
Amphibious Warfare Type
All ships having organic capability for amphibious assault and which have characteristics enabling long duration operations on the high seas.
Combat Logistics Type
Ships which have the capability to provide underway replenishment to fleet units.
Mine Warfare Type
All ships whose primary function is mine warfare on the high seas.
Coastal Defense Type
All ships whose primary function is coastal patrol and interdiction.
Mobile Logistics Type
Ships which have the capability to provide direct material support to other deployed units operating far from home base.
Support ships are not designed to participate in combat, and are generally not armed.
A grouping of ships designed to operate in the open ocean in a variety of sea states to provide general support to either combatant forces or shore based establishments. (Includes smaller auxiliaries which by the nature of their duties, leave inshore waters).
Service Type Craft
A grouping of navy-subordinated craft (including non-self-propelled) designed to provide general support to either combatant forces or shore-based establishments.
The USS Constitution, formerly IX 21, was reclassified to none, effective September 1, 1975.