We’ve all heard it before, but where does the expression 10/4 come from?
From the telecommunication world…police, fireman, CB operators all use 10 codes to shorten the length of transmitting time.
10-4 means message recieved…or OK
10-9 means repeat last message
10-20 means location
and so on and so on…
I’m sorry…maybe i shoulda said the RADIO-COMMUNICATION world.
I know that but why 10-4 and not say 20-4? Why were those specific numbers chosen?
I dont have a clue, because someone had to pick something?
I have no idea. They probably wanted to use at least a 2 digit number so they wouldnt get mixed with other numbers. For instance, when a license plate such as 103-XXX is read over the radios, it is read as one-zero-three-xray-xray-xray…so if you use a two digit code, it will limit the mixups. And i assume they chose 10 because it’s short and easy to pronounce…and it is the first two digit number. But that is just my WAG.
Here’s the best I could do on “why 10?” From http://www.911dispatch.com/information/tencode.html:
Here is a list of CB 10-codes. Most police departments in the US use 10-codes, but of course they’re different (and police-related). Here’s a sample of some police and fire 10-codes.
I don’t know why they start with “10” though … In amateur radio Q-codes are used. I don’t know why they start with “Q” though 
Arjuna34
May I venture a guess and native English speakers will decide?
Any sigle digit opening a code/sequence could be confused with a phone number sequence and/or something else. The only zip codes starting with 10… are in Manhattan, NY, and I know that they are always pronounced as one-zero-two…etc., never as ten-two-one-two or so. “Ten” is the shortest two digit number to pronounce. It still can be confused with a beginning of an address or something, but after so many years, even a rookie knows that everything starts with ‘ten’.
Many years ago I bought a $0.99 sliding card in a Radio Shack called “CB Radio Code and Slang”. It contained all 10-… combos.
Peace