This is one of a set of articles on telegraphy.

Z Code (like Q Code and X Code) is a set of operating signals used in CW, TTY and RTTY radio communication.

Distinct versions

There are at least three sets of Z codes.

1. One set of codes was originally developed by Cable & Wireless Ltd. (the Cable & Wireless Service Z code) for commercial communications in the early days of wire and radio communications.

Many of the old C&W codes are derived from mnemonics

(ZAL = alter wavelength,
ZAP = ack please,
ZSF = send faster, etc.)

The old C&W Z codes are not widely used today.

2. APCO[clarification needed] also developed a system of Z codes.[1]
3. NATO forces independently developed a later set of Z codes for military use and inter-language needs. The NATO Z codes are still in use, and are published in the unclassified document ACP-131.

There are other sets of codes internally used by Russia's military and other operating agencies.


Some example Z codes
Code Meaning Source
ZAL (...) ... I am closing down (until ...) due to ... C&W
ZAP ... Work ...
1. Simplex;
2. Duplex;
3. Diplex;
4. Multiplex;
5. Single sideband;
6. With automatic error correction system;
7. Without automatic error correction system.
8. With time and frequency diversity modem
ZBK Are you receiving my traffic clear? NATO
ZBK 1 I am receiving your traffic clear NATO
ZBK 2 I am receiving your traffic garbled NATO
ZBM 2 Place a competent operator on this circuit C&W
ZLD 2 I cannot transmit pictures C&W
ZSF ... (...) Switch off ... (except ...)
1. IFF;
2. IFF sets for 10 minutes in area denoted except for ships with the following call signs ...
ZBW ... Change to backup frequency ... C&W
ZBZ ... Measure of printability
1=Garbled / unreadable, up to 5=Perfect
ZUJ Stand by. NATO

See also


  1. ^ "Standard Z Signals". APCO International Historical Collection (