In 1872, it was decreed by maritime regulatory organizations that each ship should identify itself with four letters in the English alphabet. Thus it was a simple matter for the flagmen on each ship to spell out the ship's identification with four alphabet flags, rather than spelling out in full the long name of the ship in flag language.
When wireless stations began to proliferate right at the end of the 1800s, each wireless station adopted its own callsign, generally made up of two letters. For example, CC Cape Cod, PH San Francisco.
The signing of protocols at the Second Wireless Telegraphy Convention in Berlin took place on November 3, 1906 and these documents required that ship callsigns should consist of a group of three letters.
In 1908, the Marconi company in England required that all Marconi wireless stations on land and at sea should begin with the letter M followed by two additional letters for local identification. For example, MCC Cape Cod, MGY the SS Titanic.
On June 4, 1912 the papers were signed at the International Telegraphic Conference in London and one of the protocols was that each country throughout the world was allocated a cluster of letters in the English alphabet with which to identify their respective wireless stations. For example, wireless callsigns in Great Britain would begin with the letter B or G or M, and callsigns in France with the letter F, and American callsigns could begin with N or W, or with K beginning at KDA.
On May 9, 1913, the United States implemented its own system of callsigns (ultimately within the framework of its own internationally allocated alphabetic letters). The country was divided into nine wireless districts and thus local coverage stations were granted callsigns that comprised a number followed by two letters. Examples: 2XG New York, 3XZ Washington DC.
An international designator was added subsequently and the number of letters after the number was increased to three. Examples: W2XAD Schenectady NY, W9XAA Chicago. The X in these callsigns indicated experimental. Four letter callsigns for mediumwave stations were introduced in 1920 (KDKA), and similar four letter callsigns for shortwave were introduced in 1939 (KGEI, WRUL).
Soon after the end of World War 1, medium wave stations began to proliferate worldwide. Within continental Europe for example, Germany was granted the prefix number 4, Switzerland was granted 9, and Great Britain was granted 2 5 and 6 as the initial numbers for their callsigns. However, there seems to be no categorized cluster for the use of 2 5 and 6 in Great Britain, not in chronological order nor in geographic order.
Australia followed a similar pattern, and each state was granted a prefix number, followed by two letters for the station identification, such as for example: 2GB Sydney New South Wales, 5DN Adelaide South Australia, 7NT Northern Tasmania, 9PA Port Moresby Papua New Guinea.
(AWR-Wavescan/NWS)