47 CFR § 80.97 - Radiotelegraph operating procedures.
This section applies to ships and coast stations authorized to transmit in the band 405–525 kHz.
(a) Except for the transmission of distress or urgency signals, all transmissions must cease within the band 485–515 kHz during each 500 kHz silence period.
(b) Stations transmitting telegraphy must use the service abbreviations (“Q” code) listed in Appendix 14 to the Radio Regulations.
(c) The call consists of:
(1) The call sign of the station called, not more than twice; the word “DE” and the call sign of the calling station, not more than twice; if useful, the frequency on which the called station should reply; and the letter “K”.
(2) If the call is transmitted twice at an interval of not less than one minute, it must not be repeated until after an interval of three minutes.