10-codes

Type one or more 10-codes, like 10-4 or 10 20, and each is read out. To go the other way, type a word with no code in it, such as location or help, and every code whose meaning has that word is listed.

Ten-codes, or ten-signals, are short numbers for the things police and other radio users say most, so a busy channel stays clear. The list here is the Official Ten Signal List published by APCO, the Association of Public-Safety Communications Officials, in its ninth edition of 1 June 1974, known as Project 14.

The card stops at 10-39. 10-35 to 10-39 are reserved, with no meaning, and the card says any user may use signal numbers upward from 10-40 as best suits their own needs. That's why lists found online agree on the low numbers and fall apart above 10-34. A code like 10-50 means one thing in one county and something else in the next.

10-4 is Affirmative (OK) on the APCO list, which is how it came to mean understood. 10-20 is Location, and the 1975 song Convoy put what's your twenty, meaning where are you, into everyday American English.

Another list, often copied online and also labelled APCO, is older or expanded and runs from 10-0 to 10-100. It agrees with the 1974 card in places but not everywhere. 10-33 is Emergency there, and Help Me Quick on the 1974 card, while 10-1 is Unable to copy rather than Signal Weak.

APCO now prefers plain speech to 10-codes. Its position is that plain language over public safety radio is better than the traditional 10-codes, because agencies working together at a big incident may use the same numbers for different things. Its suggested wording for 10-4 is copy, acknowledge or understood, and for 10-20 it's my location is.