Deck code

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 1948). 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 system was adopted prior to World War I 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. Traditionally, a pendant number was reported with a full stop "." between the flag superior or inferior and the number, although this practice has gradually been dropped, and inter-war photos after about 1924 tend not to have the full stop painted on the hull. The system was used throughout the navies of the British Empire so that a ship could be transferred from one navy to another without changing its pendant number.

Pennant numbers were originally allocated by individual naval stations and when a ship changed station it would be allocated a new number. The Admiralty took the situation in hand and first compiled a "Naval Pendant List" in 1910, with ships grouped under the distinguishing flag of their type. In addition, ships of the 2nd and 3rd (i.e. reserve) fleets had a second flag superior distinguishing from which naval depot they were manned; "C" for Chatham, "D" for Devonport, "N" for Nore and "P" for Portsmouth. Destroyers were initially allocated the flag superior "H", but as this covered only one hundred possible combinations from H00 to H99 the letters "G" and "D" were also allocated. When ships were sunk, their pendant numbers were reissued to new ships.

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