Ten-codes are usually traced to Charles Hopper of the Illinois State Police, who is credited with beginning work on them in 1937, and to the Associated Police Communication Officers, the organization now called APCO International, which carried them to agencies across the country and later revised them. The reason most often given for the format was mechanical, because the radios of the period tended to lose the first syllable of a transmission, so a throwaway number went in front of the message. This piece follows that origin, the 1974 APCO list, and the local drift that came after it.

1937, about 1940, and 1974: what each date marks

The ten-code story is usually told as if one man wrote one list in one year and the country adopted it, and the record is messier than that. The year 1937 is the one APCO and most published histories give for when Charles Hopper began developing the codes in Illinois. The association he worked through had formed only a couple of years earlier, in 1935, under the name Associated Police Communication Officers. A list then circulated to APCO’s members, and most accounts place that circulation around 1940, although the exact year varies between sources and I have not seen the original publication, so I am not giving a firmer date than that.

The third date is 1974, the year most accounts give for APCO’s revision and expansion of the list. The 1974 version is the one most people mean when they say “the APCO ten-codes,” and it is the version reproduced in training handouts, on websites and in the backs of old dispatcher manuals. It came more than three decades after the first circulation, which means the codes had already been in use, and already been modified locally, for a full generation of radio operators before APCO tried to restate them in one place.

Keeping those dates separate matters because each one marks a different kind of event. The first is local invention, the second is adoption by a professional association with no legal authority over anyone, and the third is an attempt at standardization that arrived after the habits were set. Readers who need exact publication details for a citation should go to APCO International’s own historical material and archive of its bulletin, which are the primary sources for the association’s side of the story, instead of relying on secondary retellings, mine included.

Pontiac, Illinois: what the record says about Charles Hopper

Charles Hopper is described in most accounts as the communications director, or the radio official in charge, for Illinois State Police District 10, which was headquartered in Pontiac. I am keeping his title at that level of description because sources do not agree on his exact position and I could not tie it to a primary document. What the accounts agree on is that he worked on the operating side of state police radio, where the daily problem was getting accurate messages between a district station and troopers on long stretches of highway, and that he is the person APCO and later histories credit with starting the ten-code list.

State police radio in the 1930s was young, and its operators were a mixed group. Some were trained radiomen with commercial or amateur backgrounds, while others were troopers and clerks who had been handed a microphone. A district station handled traffic for cars spread over a large territory, the channel was shared, and the equipment was unforgiving. In that setting a short, fixed vocabulary had obvious appeal, because it reduced the time each message occupied the channel and gave operators of uneven skill the same words for the same situations.

I am not claiming that Hopper’s list was the first numeric shorthand any police radio operator ever used. Departments had private codes and habits of speech before 1937, and it would be surprising if none of them used numbers. What the record supports is narrower, and it is enough: Hopper’s list is the one that APCO took up and circulated, and it became the ancestor of the ten-codes still heard on many channels today. The retellings that turn him into the inventor of all police radio shorthand go further than the sources do.

The lost first syllable: what the radio was doing at key-up

The reason APCO and most histories give for the format is that early radio systems tended to lose the first syllable of a transmission, so an operator who began speaking the moment he pressed the button would have the start of his message cut off. Putting “ten” in front of the number meant that if anything was lost it was the throwaway word, and the number that carried the meaning came through. The listener also heard “ten” or a fragment of it and knew that a code was coming, which primed him to catch the next word.

The equipment of the period makes that explanation plausible, although I want to be careful about how far I take it. Mobile transmitters of the 1930s generally drew their high plate voltage from a dynamotor or a vibrator power supply, and pressing the microphone switch typically energized relays that moved the antenna from the receiver to the transmitter and started that supply. A dynamotor needs a fraction of a second to come up to speed, and relays need time to pull in and settle, so the carrier did not reach full strength the instant the switch closed. At the receiving end, an operator who had been listening to noise or doing paperwork needed a moment before he was actually attending to the speaker.

Those are the general behaviors of the equipment and the people of that era. I have not seen an account in Hopper’s own words that names which component or which delay he was working around, and I am not going to assign the clipping to one cause as though the record settled it. The commonly repeated version of the origin says only that the first syllable was being lost and that the number prefix was the fix. Anyone briefing this history is on solid ground repeating that version as the reason APCO and the histories give, and on weaker ground if they present a specific engineering diagnosis as Hopper’s finding.

Numbers before Hopper: telegraph codes and the Q-signals

Numeric and letter brevity codes were old by 1937, and Hopper was applying an established idea to voice radio. Telegraph operators had used numeric shorthand since the nineteenth century, and Western Union’s “92 Code” of 1859 is the usual example. Amateur radio still carries a survivor of that era, because “73,” meaning best regards, comes down from telegraph practice. Radiotelegraphy then developed the Q-signals, three-letter groups beginning with Q that stood for standard questions and answers, which were adopted internationally under the radiotelegraph regulations in the years before the First World War so that operators who shared no language could still exchange routine information.

Those earlier codes solved problems that a voice operator in 1937 also had. A fixed code takes less channel time than a spoken sentence, it means the same thing every time it is used, and it relieves the sending operator of composing a message under pressure. For a receiving operator writing a log by hand, a two-word code is also faster to copy and less likely to be garbled than a paraphrase. The ten-code borrowed all of that and added the prefix, which addressed a problem Morse operators did not have, since a clipped dot at the start of a telegraph transmission was a different matter from a clipped syllable in speech.

A later justification sometimes attached to the ten-codes is that they kept the public and criminals from understanding police traffic. I have not found that purpose in the origin accounts I consider reliable, and it sits awkwardly with the fact that APCO circulated the list to agencies across the country and that versions of it were reprinted widely in later decades. I treat secrecy as a claim made about the codes after the fact, unsupported as their original purpose, and I would not teach it as part of the 1937 story without a primary source that says so.

From one district to a national association: how the list spread

APCO in its early years was an association of police radio people, which is what its original name, Associated Police Communication Officers, describes. It gave a radio official in one Illinois district a way to reach his counterparts in other states, through meetings and through the association’s bulletin, and that is the channel through which Hopper’s list traveled. The association has since broadened to cover all of public safety communications and renamed itself the Association of Public-Safety Communications Officials, keeping the APCO initials, but in the late 1930s and 1940s its audience was police radio.

APCO had no authority to require anything. It was a professional body, and adoption of its list was voluntary, which shaped everything that happened afterward. An agency could take the list whole, take part of it, renumber it, or add codes for situations its operators faced that the list did not cover. Accounts describe the early list as shorter than the later versions, which is consistent with a set of codes written by one district for its own traffic and then picked up by others who extended it to suit themselves.

The police origin also explains the shape of the vocabulary. The codes were written for law enforcement radio traffic, so they covered signal reports, unit status, and the incident types a state police district dealt with. Fire and EMS agencies that adopted ten-codes later had to fit their own operations into a vocabulary built for another discipline, and many of them added their own codes or wrote separate lists, which is one of the earliest sources of the variation that the 1974 revision would later try to reduce.

The 1974 revision: what APCO’s list standardized and what it could not

Most accounts give 1974 as the year APCO revised and expanded the ten-code list, and the result is the version most often reproduced as “the APCO list.” Its low numbers are mainly about the radio contact itself and the status of the unit. The meanings that appear consistently in reproductions of that list include 10-1 for unable to copy or poor signal, 10-2 for signal good, 10-4 for acknowledgment, and 10-9 for a request to repeat, and 10-20 for location is probably the best-known code outside the profession. Higher numbers cover incident types and requests that reflect the law enforcement work the list was written for.

I am not reproducing the full list here, because the versions in circulation differ in places and I cannot confirm which reproduction matches APCO’s 1974 publication word for word. Agencies and individuals have copied the list for decades, sometimes with local codes mixed in, and a list found on the internet labeled “APCO ten-codes” may be partly someone’s department list. Anyone who needs the authoritative text should obtain it from APCO International instead of from a reprint.

The revision had the same limit the original circulation had, which is that APCO could publish a list and recommend it but could not make anyone use it. By 1974 many agencies had been running their own versions for thirty years, their dispatchers and officers had learned those versions, and their written procedures and logs were built around them. A national list arriving into that environment could pull new systems and new agencies toward a common vocabulary, and it gave everyone a reference point, but it could not erase local meanings that were already in daily use.

Local drift after 1974: the same number with different meanings

Drift continued after 1974 for the same reasons it existed before, because adoption was voluntary and local needs kept producing new codes. A few well-known examples show how far it went. The New York City Police Department is widely reported to use 10-13 for an officer needing assistance, while the commonly reproduced APCO list assigns 10-13 to a weather or road report and puts emergency at 10-33. The California Highway Patrol uses its own 11-series codes, and 11-99, its call for an officer needing help, is well enough known that a foundation supporting the agency’s families takes its name from it. Many California agencies also call incidents by penal code section, so that 211 means robbery, which is a different system entirely.

Fire and EMS lists drifted as well, and so did the lists of individual counties and cities. A code that meant a traffic crash in one county could mean something unrelated a few miles away, and a code reserved for a dangerous situation in one agency could be routine administrative traffic in its neighbor. None of this was careless in the sense of operators ignoring a rule, because there was no rule binding them, and each local change made sense to the people who made it. The cumulative effect was a vocabulary that sounded national and was in practice regional or local.

The codes also left the dispatch room. The television series Highway Patrol in the late 1950s is commonly credited with putting “10-4” into general American speech, and the citizens band radio boom of the mid-1970s spread ten-codes to drivers and hobbyists, with songs like C. W. McCall’s “Convoy” in 1975 carrying the vocabulary further. CB users developed their own ten-code lists, which differed from APCO’s in a number of meanings. The CB peak came at roughly the same time as APCO’s 1974 revision, but I have seen nothing in the record connecting the two, and I would not suggest one influenced the other. The consequences of all this drift for multi-agency operations are covered in the plain language article elsewhere in this section, and I am not reopening that argument here.

Clipping today: trunking grants, digital delay, and the talk permit tone

The problem Hopper’s prefix addressed never went away, although its causes changed. On a trunked system, pressing the button sends a channel request to the controller, which must grant a voice channel before the radio transmits any audio, and on a digital system such as P25 the voice is also passed through a vocoder and, on a simulcast system, timed across multiple sites. Each of those steps takes time. A user who starts speaking at the instant of key-up on a modern system can lose the first word or more in the same way a 1930s trooper did, even though the delay now comes from network signaling and processing and not from a dynamotor spinning up.

Conventional analog systems have their own versions of the delay. A repeater has to detect the incoming signal and its tone, key its transmitter, and get its signal to the receiving radio, which must then open its own squelch, and each of those stages can shave off the start of a transmission. The length of the delay varies with the system, the equipment and the configuration, so I am not giving figures. The radio shop can measure it on your own system, and that measurement is more useful than any general number I could offer.

The modern answer is the talk permit tone, the short tone a trunked radio sounds when the channel has been granted, together with training users to wait for it before speaking. That answer depends on the tone actually being enabled in the codeplug and on users actually waiting. Some fleets have the tone turned off or turned down because users find it annoying, and some users never learned what it means, so they talk over it. The ten-code prefix was a way of building tolerance for clipping into the language itself, and agencies that have moved away from ten-codes for other reasons still have to handle clipping through configuration and training, because a message that begins before the system is ready is lost regardless of what vocabulary it was spoken in.

What to do at your agency

• The radio administrator pulls the current codeplug templates for every subscriber model in the fleet this month, confirms that the talk permit tone is enabled and set to an audible level, and writes down any model or template where it is off.
• The radio shop records ten test transmissions on each system or talkgroup type in use, with the tester speaking immediately at key-up, and gives the training officer the recordings showing how much of the first word, if any, was lost.
• The training officer adds a key-up exercise to the next new-member radio session, in which each member transmits once without waiting for the tone and once after it and then hears both recordings played back.
• The records custodian or communications director locates the document that established the agency’s current code list, notes its date and who adopted it, and files that information with the radio procedures so the list has a traceable source.
• The dispatch supervisor compares the agency’s code list against the lists of the two or three agencies it most often works with on mutual aid, and writes a one-page note of any code that has different meanings between them for the next regional dispatch meeting agenda.
• The training officer revises the radio history portion of new-member training so that it credits Hopper and APCO with the list that spread nationally, gives the lost-syllable explanation as the reason usually given, and removes any statement that the codes were created for secrecy.

Takeaways

• APCO and most histories credit Charles Hopper of the Illinois State Police with beginning work on the ten-codes in 1937, while he was working in state police radio in a district headquartered at Pontiac.
• APCO, founded in 1935 as the Associated Police Communication Officers, carried Hopper’s list to agencies across the country, with circulation usually placed around 1940, although accounts differ on the exact year.
• The reason usually given for the format is that radios of the period lost the first syllable of a transmission, so the throwaway “ten” absorbed the clipping and the number carrying the meaning came through intact.
• Ten-codes applied to voice radio an idea that telegraph numeric codes and radiotelegraph Q-signals had already established, which was a short, fixed vocabulary that saved channel time and meant the same thing every time.
• The claim that ten-codes were designed for secrecy does not appear in reliable origin accounts and should not be taught as part of the 1937 story.
• APCO revised and expanded the list in 1974, but because adoption was always voluntary, the revision could not override local meanings that agencies had used for three decades.
• Local drift produced well-known conflicts, such as NYPD’s reported use of 10-13 for an officer needing assistance and the California Highway Patrol’s own 11-series codes, so a code’s meaning depends on whose list is in use.
• Clipping at key-up still happens on trunked, digital and repeater systems, and the modern fix is an enabled talk permit tone and users trained to wait for it.