ACP 131

Lore & Background

The original edition of ACP-131 was published by the U.S. military during the early years of radio telegraphy for use by radio operators using Morse Code on continuous wave (CW) telegraphy. It became especially useful, and even essential, to wireless radio operators on both military and civilian ships at sea before the development of advanced single-sideband telephony in the 1960s.

Radio communications, prior to the advent of landlines and satellites, was always subject to unpredictable fade outs caused by weather conditions, practical limits on available emission power, radio frequency, type of emission, type of transmitting antenna, signal waveform characteristics, modulation scheme, receiver sensitivity, presence of atmospheric reflective layers, time of day, and numerous other factors. Because of these factors, which often limited transmission time to only several hours or minutes a day, it was found necessary to keep each wireless transmission as short as possible. Using ACP-131 codes, a question like 'How are you receiving me?' could be phrased simply 'INT QRK', resulting in much more efficient use of circuit time.

According to ACP-125(F), paragraphs 103 and 104, Q codes are authorized for both civilian and military use, and for communications between the two, while Z codes are authorized for use only among military stations. Some assert that the use of Q codes and Z codes was not intended for use on voice circuits, where plain language was speedy and easily recognizable, but some voice operators, such as amateur radio operators, find it convenient or traditional to carry over some Q codes to voice exchanges, such as 'QSL', 'QRK', 'QTH'.

Reader's Guide

ACP-131's significance lies in its role as the controlling publication for Q and Z codes, which were developed to address the severe limitations of early radio telegraphy. Before landlines and satellites, radio communications were plagued by unpredictable fade outs and limited transmission windows, often only a few hours or minutes per day. The codes allowed operators to replace lengthy plain-language queries—such as 'How are you receiving me?'—with brief code groups like 'INT QRK', dramatically reducing circuit time and operator fatigue. The response 'QRK 5' for 'loud and clear' further streamlined exchanges. This efficiency was critical on shared CW radio circuits where many operators waited to transmit. The codes also enabled communication between ships of different nations during NATO exercises when no common language existed. While originally designed for Morse Code telegraphy, some Q codes have been carried over to voice exchanges by amateur radio operators, though the article notes that some assert this was not the intended use. The distinction between Q codes (authorized for civilian and military use) and Z codes (military-only) remains a key feature of Allied communications doctrine.

Did You Know?

• Using ACP-131 codes, the question 'How are you receiving me?' could be shortened to 'INT QRK'.
• According to ACP-125(F), Z codes are authorized for use only among military stations, while Q codes are authorized for both civilian and military use.