In the Royal Navy and other navies of Europe and the Commonwealth of Nations , ships are identified by pennant number an internationalisation of pendant number , which it was called before Historically, naval ships flew a flag that identified a flotilla or type of vessel. For example, the Royal Navy used a red burgee for torpedo boats and a pennant with an H for torpedo boat destroyers. Adding a number to the type-identifying flag uniquely identified each ship.
In the current system, a letter prefix , called a flag superior , identifies the type of ship, and numerical suffix , called a flag inferior, uniquely identifies an individual ship. Not all pennant numbers have a flag superior.
The Royal Navy first used pennants to distinguish its ships in with a proclamation that all of his majesty's ships must fly a union pennant. This distinction was further strengthened by a proclamation in which forbade merchant vessels from flying any pennants. The system of numbering pennants was adopted prior to the First World War to distinguish 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.
During the First World War, pennant numbers were changed on a regular basis, with only those for ships in Home waters centrally controlled; those on foreign stations were allocated on a local basis. However, in November a new navy-wide system was introduced, with the intention that ships should now carry a permanent number. In most cases, plain numbers were given to capital ships and cruisers, and ones with flags-superior to smaller ships.
While the numbers allocated to major warships except for a few older vessels would generally remain stable into the s, destroyers were subject to further revisions, stability being reached in January The next major revision took place in the late s, when the volume of new construction was such that insufficient 'spare' numbers were now available for new ships. Accordingly, older cruisers had the flag-superior 'I' added in front of their existing plain numbers; as a result, submarines, which had previously used 'I', were given a new scheme of flags-inferior.
