In the modern Royal Navy and other navies of Europe and the Commonwealth, ships are identified by pennant numbers (sometimes referred to as pendant numbers). The system was adopted after World War I to distinguished between ships with the same or similar names, to reduce the size and improve the security of communications and to assist recognition when ships of the same class are together. A pennant number consists either of a letter (known as a flag superior) followed by a number, or just a number (as not all pennants have a flag superior). Typically, all the ships in a class share the same flag superior, for example many aircraft carriers of the Royal Navy have flag superior "R". The system was used throughout the navies of the Commonwealth so that a ship could be transferred from one navy to another without changing its pennant number.

When ships were sunk, their pennant numbers were reissued to new ships. In a few cases, the flag superior for whole ship classes changed while the numbers stayed the same. For example, many F-class destroyers of the Royal Navy changed from "F" to "G" in 1940.

See also

• Hull classification symbol
• Ship prefix