In broadcasting and radio communications, a call sign (also known as a call name or call letters—and historically as a call signal—or abbreviated as a call) is a unique designation for a transmitting station. In North America they are used as names for broadcasting stations. A call sign can be formally assigned by a government agency, informally adopted by individuals or organizations, or even cryptographically encoded to disguise a station's identity.
The use of call signs as unique identifiers dates to the landline railroad telegraph system. Because there was only one telegraph line linking all railroad stations, there needed to be a way to address each one when sending a telegram. In order to save time, two letter identifiers were adopted for this purpose. This pattern continued in radiotelegraph operation; radio companies initially assigned two-letter identifiers to coastal stations and stations aboard ships at sea. These were not globally unique, so a one-letter company identifier (for instance, 'M' and two letters as a Marconi Station) was later added. By 1912, the need to quickly identify stations operated by multiple companies in multiple nations required an international standard; an ITU prefix would be used to identify a country, and the rest of the call sign an individual station in that country.
One of the earliest applications of radiotelegraph operation, long predating broadcast radio, were marine radio stations installed aboard ships at sea. In the absence of international standards, early transmitters constructed after Guglielmo Marconi's first trans-Atlantic message in 1901 were issued arbitrary two-letter calls by radio companies, alone or later preceded by a one-letter company identifier. These mimicked an earlier railroad telegraph convention where short, two-letter identifiers served as Morse code abbreviations to denote the various individual stations on the line (for instance, AX could represent Halifax). 'N' and two letters would identify US Navy; 'M' and two letters would be a Marconi Station. On April 14, 1912, the RMS Titanic station MGY, busily delivering telegram traffic from ship's passengers to the coastal station at Cape Race, Newfoundland (call sign MCE), would receive warnings of ice fields from Marconi stations aboard the M.V. Mesaba (call sign MMU) and the S.S. Californian (call sign MWL). Its distress call CQD CQD CQD CQD CQD CQD DE MGY MGY MGY MGY MGY MGY position 41.44N 50.24W would be answered by a station aboard the Carpathia (call sign MPA). Later that same year, an international conference standardised radio call signs so that the first two letters would uniquely identify a transmitter's country of origin.