Marine Callsigns are short identifiers used to uniquely identify a vessel during radio-communications. Recreational and small commercial vessels often are not assigned callsigns, but rather use their vessel name for communications. Larger vessels will have callsigns assigned by their flag state.
Early callsigns were tied to company-owned operation sets onboard vessels, being internal codes of generally two characters. As proliferation increased, this was expanded to include a company identifier (i.e., "M" for the Marconi company, followed by the station identifier). Today, under international agreements, marine callsigns consist of two parts - a national identifier prefix, followed by the unique station identifier. Numbers and letters may be intermixed.