Procedural Signals include: CQ—Calling any station, AR—over, end of message, K—go, invite any station to transmit, KN—'X' go only, invite a specific station to transmit, BK—invite receiving station to transmit, R—all received OK, AS—please stand by, SK—end of contact (sent before call), CL—going off the air.
If you have ever said 73, you may not have known that this is taken from Western Union Company’s 92 Code that dates from the telegraph era, and were the equivelant of the Q-codes that developed for use in Morse Code. The codes were first published in the April 1857 National Telegraphic Review and Operators’ Guide, and then defined as the “Standard 92 Code” by Western Union in April 1859. The codes still in use are: 73 for “Best regards” and 88 for “Love and kisses”.
The earliest documentation K6WEB has been able to find on the Web of these WU rules is the The Western Union Telegraph Company Rules, Regulations, and Instructions, published in 1866, though this document does not list the 92 Code. http://www.signalharbor.com/73.html 1857 US 'National Telegraphic Review & Operator's Guide' - Numeral Codes. It is also the origin of using SK for the end of a message, since the 92 Code for “the end, no more” was 30, which when run together, sounds like SK.
Derived from the British Post Office’s two-letter codes (RA to RZ and SA to SF) published in the 1908 Instructions to Wireless Telegraphists, Q codes were codified by the ITU in 1912 by adding Q to these existing codes and extending the range (QRA to QRZ and QSA to QSX). Q codes were officially adopted by the U.S. military in 1942, which explains there prevelance today.