This persisted until the Navy's modern hull classification system's introduction in 1920, which included Coast Guard ships and craft.

Like the U.S. Navy, the United States Coast and Geodetic Survey – a uniformed seagoing service of the United States Government and a predecessor of the National Oceanic and Atmospheric Administration (NOAA) – adopted a hull number system for its fleet in the 20th century.

The Naval Vessel Register maintains an online database of U.S. Navy ships showing which symbols are presently in use.

After World War II until 1975, the U.S. Navy defined a "frigate" as a type of surface warship larger than a destroyer and smaller than a cruiser.

If a ship's hull classification symbol begins with "W", it is a commissioned cutter of the United States Coast Guard.

The National Oceanic and Atmospheric Administration (NOAA), a component of the United States Department of Commerce, includes the National Oceanic and Atmospheric Administration Commissioned Officer Corps (or "NOAA Corps"), one of the eight uniformed services of the United States, and operates a fleet of seagoing research and survey ships.

The research ships, which include oceanographic and fisheries research vessels, are given hull numbers beginning with "R", while the survey ships, generally hydrographic survey vessels, receive hull numbers beginning with "S".

Although considerably changed in detail and expanded over the years, this system remains essentially the same as when formally implemented in 1920.

It is a very useful tool for organizing and keeping track of naval vessels, and also provides the basis for the identification numbers painted on the bows (and frequently the sterns) of most U.S. Navy ships.

During the course of the next thirty years, these same numbers were combined with filing codes used by the Navy's clerks to create an informal version of the system that was put in place in 1920.

It is important to understand that hull number-letter prefixes are not acronyms, and should not be carelessly treated as abbreviations of ship type classifications.

Contrary to popular belief, the "CV" hull classification symbol does not stand for "carrier vessel".

"CV" derives from the cruiser designation, with one popular theory that the V comes from French voler, "to fly", but this has never been definitively proven.

Surface combatants are ships which are designed primarily to engage enemy forces on the high seas.

While some classes, including all diesel-electric submarines, are retired from USN service, non-U.S. navies continue to employ SS, SSA, SSAN, SSB, SSC, SSG, SSM, and SST types.

Patrol combatants are ships whose mission may extend beyond coastal duties and whose characteristics include adequate endurance and seakeeping, providing a capability for operations exceeding 48 hours on the high seas without support.

Mobile logistics ships have the capability to provide direct material support to other deployed units operating far from home ports.

For ships with civilian crews (owned by and/or operated for Military Sealift Command and the Maritime Administration), the prefix T- is placed at the front of the hull classification.

Although aircraft, pre-World War II rigid airships were commissioned (no different from surface warships and submarines), flew the U.S. ensign from their stern and carried a United States Ship (USS) designation.

Rigid airships: Lighter-than-air aircraft (e.g., blimps) continued to fly the U.S. ensign from their stern but were registered as aircraft: United States Navy Designations (Temporary) are a form of U.S. Navy ship designation, intended for temporary identification use.

On the other hand, a U.S. Navy vessel, such as the yacht in the example above, already in commission or service, might be desired, or found useful, for another need or purpose for which there is no official designation.

Numerous other U.S. Navy vessels were launched with a temporary, or nominal, designation, such as YMS or PC, since it could not be determined, at the time of construction, what they should be used for.

Many of these were vessels in the 150 to 200 feet length class with powerful engines, whose function could be that of a minesweeper, patrol craft, submarine chaser, seaplane tender, tugboat, or other.

The U.S. Coast Guard considers any ship over 65 feet in length with a permanently assigned crew, a cutter.