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What Does 10-4 Mean

The 10-code “10-4” Meaning: Same as “Message received” or “Acknowledged”


Despite the fact that it is no longer used by the police in real life, the code “10-4” is still very popular in the film industry. If you’re a fan of police dramas, you’ve probably heard it before and wondered what it means.

Usually, it’ll show up in a dramatic scene like a car chase or just before an important arrest, when some harried police officer uses it to respond to a radio call from the dispatcher.

If you don’t know what it means, don’t feel bad – many of us watch TV without Googling every word or phrase we don’t understand. This article will make sure you understand all things 10-4.

Decoding 10-4

All it is is an old police code for the response, “Message received” or “Acknowledged.” It was, and still is in some industries, used to let someone know that their message has been heard and understood.

So if someone is explaining something to you and it finally starts to make sense, instead of just saying, “Got it,” or “I understand,” you can say, “10-4!” So much cooler than boring old English.

Here are some other words and phrases with similar meanings to 10-4:

  • “I got you.”
  • “I hear you”
  • “I understand.”
  • “Copy that”
  • “Roger that”
  • “Received, loud and clear”

It’s important to note that saying 10-4 doesn’t mean you will do what the message instructs you to do, just that you have heard and understood it.

If you wanted to let them know that you have received their message AND will do what it says, you could say, “10-4 – wilco,” which means, I understand your message and I will comply with your instructions.”

How to Respond to 10-4

If someone says “10-4” in conversation with you, there is really no need to reply. They are simply saying that they understand you. If there is more information they need, you can supply it before ending the conversation, or if it feels awkward if you don’t respond, you can say “10-4” back to them. 

Sometimes, if the police were talking on the radio and someone responded to an instruction with 10-4, the other speaker might say, “10-4, over and out.” This means that both parties understand each other and there’s nothing more to say, so the person is signing off or leaving the conversation.

The History of 10-4

Back in the 1930s when radio was very new and far from perfect, police communications were problematic, with lots of important chatter being lost to the static.

In high-stress situations, instant and clear communication can be the difference between life and death, so those early cops needed to come up with a better way.

A radio expert at the Illinois Police Department named Charles Hopper had an idea. He proposed that they assign numbers to the words and phrases they used most often in their daily patrols.

He assigned numbers between 1 and 99 to have specific meanings, which allowed the officers to get their message across more quickly. It also meant that the public couldn’t understand the police broadcasts.

The system was immediately adopted by the police department, but there was a problem. In those days, radio was powered by a dynamotor – a tiny generator – which despite being pretty incredible technology for the time, was a little slow to start up.

The device would take a fraction of a second to spool up when someone called in, so the first syllable of what they said was often cut off.

The police department tried to get the officers to take a brief pause before speaking, but when you’re chasing a criminal or being shot at, it’s kinda hard to remember something like that, and communication didn’t improve enough with the use of the new codes. 

To solve the issue, they decided to prefix the codes with the number “10.” With the officers saying it before the actual code, the little motor-generator had the time it needed to spin up, and nothing important was lost in the communication. The new code was called the Ten Code, 10-code, or Ten Signals.

Evolution of the 10-Code

As with any secret, the 10-code couldn’t stay hidden in the police department forever. Eventually, the meanings of the individual numbers got out, and it slowly became public knowledge.

The code started appearing in movies and TV shows at some point between the 1950s and the 1970s, and many radio users realized how useful it could be for them.

There were lots of people who used radio for their jobs, like long-haul truckers and emergency workers, and there were plenty of CB (Citizens Band) radio enthusiasts who felt that they could make the code work for them. They adapted it to suit their own needs, and the 10-code began its slow evolution. 

Today, there are numerous versions of the 10-code in existence, and many of the meanings are no longer what they were almost 100 years ago.

For example, in 1940, “10-12” meant “Officials or visitors present,” while most users now use it for “Stand by.” A lot of the codes are still in regular use by civilians, and some of them you will already have heard. A good example is the code for location; “20” – as in, “What’s your 20?” Which means, “Where are you?”

These days, we have much better radio signals, the 10-codes are no longer private, and they are just one more thing young recruits need to learn, so the Federal Government decided to revert to normal speech for police radio communications.


The code, “10-4” simply means, “Acknowledged” or “Message received.” If someone says “10-4” to you, they are simply telling you that they understand you, and there is usually no need to respond to them.

“10-4” comes from the old police code, called the 10-code, that was used for clear and private communication starting in the 1930s. Due to the imperfections of radio back then, the code was needed to prevent the loss of important communications between officers and their dispatchers. 

Over time, the code was discovered by the population and they quickly adopted it and adapted it to suit their own needs. It is still used today by truckers and emergency workers, though the police department stopped using it a few years ago.

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