Morse code was later made the standard by the International Telecommunication Union (ITU). Morse's original code specification, largely limited to use in the United States and Canada, became known as American Morse code or "railroad code". American Morse code is now seldom used except in historical re-enactments. In aviation, pilots use radio navigation aids. To ensure that the stations the pilots are using are serviceable, the stations transmit a set of identification letters (usually a two-to-five-letter version of the station name) in Morse code. Station identification letters are shown on air navigation charts. For example, the VOR-DME based at Vilo Acuña Airport
Morse code abbreviations only. To station S1 from station S2. Roger (R) Thank you (TU) see you again (CUAGN) Best regards (73) To station S1 from station S2: Signing off. In International Morse code there is no "distinct" dot-dash sequence defined only for the mathematical equal sign [=]; rather the same code ("–···–" or "dah di di di dah") is shared by "double hyphen" [=] and the procedural sign for "section separator" notated as . It is fairly common in the "Recommended International Morse Code" for punctuation codes to be shared with prosigns. For example, the code for plus or cross [+] is