In 1817 the Board of Trade formed a committee to devise a better method of ship identification. This was to replace the earlier system devised by Captain Maryatt based on numerals which had proved to be too cumbersome. The result was a four letter code to be allocated to each ship and was to be flown as flags in a single hoist. The codes were listed in the Mercantile Navy List together with details of the vessel, including the registration number, making it easy to identify the vessel by reference to the list. It enabled officers at Signal Stations on shore and masters of ships at sea to signal and report passing ships. When a vessel was sold or subject to a name change, it retained the same signal letters. Only when a ship was disposed of or lost were the signal letters reallocated. The system was available to all mercantile ships and the list, revised and published anually, contains vessels from 1 ton grt. upwards. Signal letters were also used in radio communication and during WW2 U-boats monitored the 600/800 meter band for distress signals to identify victims by their callsigns. The system continued to be used until 1947 when it was replaced by the modern alphanumeric call sign.
In accordance with the International Code of Signals, the first letter or the first and second letters of a visual signal group always indicated the nationality of the ship and the following table shows the allocation of initial letters so far as British ships were concerned :- [TABLE]