In our modern day world, we use the phrase “10-4” (ten four) to mean “I heard and understand what you said.”
That is actually the meaning of the phrase in its origin, too. Back in the early days of radio use, policemen realized that speed and encrypted messages were best when trying to communicate with each other.
An employee of the Illinois State Police, Charlie Hopper, took a series of numbers and assigned each number its own meaning. For example, the number 1 meant “bad connection” while the number 9 meant “say again or repeat.”
This shortened method of communication was quickly adopted, but there was a slight problem. Due to the technology available at the time, early radios sometimes did not effectively transmit the first part of a message. By adding the word “ten” in front of the coded number, radio users were guaranteed to get the important part of the message.
Thus, “10-4” was used to communicate the phrase “I acknowledge.”
During the advent of television in the 1950s, households began to hear the phrase used in police dramas. This helped the public to understand the meaning of the phrase. Later on, CB radio users adopted the same codes and this spread the popularity of the phrase “10-4” even further.
By the 1980s, the phrase “10-4” was in wide use by the entire country and everyone understood its meaning without difficulty.
This phrase (and other ten codes) began to be phased out of use in the early 2000s.