Z code
Z Code is a set of operating signals used in CW, TTY and RTTY radio communication. There are at least three sets of Z-codes: one originally developed by Cable & Wireless Ltd. for commercial communications in the early days of wire and radio communications, one independently developed by NATO forces later for military needs and use, and one developed by APCO. The NATO Z Code is still in use today, and is published in the unclassified document ACP-131. There are other sets of codes internally used by Russia's military and other operating agencies. The old C&W Z Codes are not widely used today.
| Code | Meaning | Source |
| ZAL | I am closing down 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 | C&W |
| ZBK | Are you receiving my traffic clear? | NATO |
| ZBK1 | I am receiving your traffic clear | NATO |
| ZBK2 | I am receiving your traffic garbled | NATO |
| ZBM2 | Place a competent operator on this circuit | C&W |
| ZLD2 | I cannot transmit pictures. | C&W |
| ZSF | Switch off.... | C&W |
| ZBW no. | Change to backup frequency no. | C&W |
| ZBZ. | Measure of Printability | C&W |
| ZUJ | Stand by. | NATO |
A lot of the old C&W codes are derived from mnemonics (ZAL = alter wavelength, ZAP = ack please, ZSF = send faster, etc.