What Does 10-4 Mean? Only the 4 Carries the Message

“10-4 Acknowledgement.” It sits fourth on a revised list of 70 police radio signals printed on page eight of The APCO Bulletin for January 1940, and the page says nothing more about it.

No signal on the list means yes. The nearest thing to one on the page is the quiz printed under the last code, “CW Op - Can You Pass This Exam?”, where answers “in the affirmative” count as promises.

The rest of the list is stranger. 10-32 is “Is drunkometer available?”, 10-14 is “Convoy or escort” and 10-82 is “Reserve room with bath at hotel for officer.” 10-12, “Officials or visitors present,” was later drafted into the folklore about why police are called 12.

What does 10-4 mean?

10-4 means “message received.” It is a police radio signal from the ten-code list that the Associated Police Communication Officers, or APCO, adopted nationally in 1940, and its official meaning was “Acknowledgement”: I heard you, not yes. Everyday speech later widened it into “okay” and “will do,” and APCO redefined it as “Affirmative (Ok)” in 1974.

The ten is an alert and the number is the message

APCO set out its reasoning in a 1967 operating manual, reprinted in a 1968 federal training manual for police radio personnel: “The purpose of the TEN SIGNALS is two-fold: to achieve reliability and speed.”

Reliability comes from the front half, where the “10-” is “an euphonic ‘alert’ attesting to the fact that information is about to follow.” The number is “the information content,” several words squeezed into one figure, and numbers “are more easily read through the noise that is ever present in a two-way radio system.” Wireless had applied the same test to its distress call decades earlier: SOS stands for nothing, and the 1906 Berlin treaty that adopted it prescribes only three dots, three dashes and three dots, a rhythm chosen as “easy to transmit and difficult to mistake.”

In APCO’s own account, 10-4 has two parts. The “10-” only tells the listener to pay attention, and the 4 carries the entire message.

The manual also sets a grammar rule: “A TEN SIGNAL is complete in itself.” The correct form is “10-4, Rockford.” The incorrect one is “I am 10-4 on that information, Rockford.” Anyone who has said “I’m 10-4 on that” has made the manual’s textbook error, and usage keeps outvoting rules like this one, including the classroom ban on starting a sentence with but.

The warm-up story has no source attached

The popular explanation is mechanical. Wikipedia’s article on ten-codes says 1930s police radios ran on a motor-generator called a dynamotor, which “took from 1/10 to 1/4 of a second to ‘spin up’ to full power,” so saying “ten” first gave the transmitter time to reach full strength. That passage carries no citation of its own.

Neither the 1940 Bulletin nor the 1967 manual mentions a warm-up. The Bulletin gives no reason for the prefix, and the manual’s alert sentence points back to the custom of a calling station naming itself first, a habit it dates to a time when “static and noise on the low frequencies then in use made it necessary for the called station to be alerted that something was coming through the racket.” Until a period source mentions the dynamotor, the warm-up story is plausible but unsourced.

APCO itself turned 10-4 into a yes in 1974

The standard list printed in the APCO Bulletin of April 1957, prepared by the National Operating Procedure Committee in 1955, gave 10-4 as “Message received,” the sense Cambridge’s dictionary still gives it. The 1967 manual went back to “Acknowledgement,” which its glossary defines as the act by which one operator “signifies to another that a message has been received.” That list ran to 10-99, with 10-74 for “Negative” and no affirmative.

A February 1968 Bulletin column, reprinted in the training manual, coached operators to keep the word “I” off the air: “instead of ‘I will do it,’ substitute ‘10-4.’ Instead of ‘I don’t think so,’ say ‘10-74.’” In 1974 APCO made the pairing official: its Project 14 brevity code lists 10-4 as “Affirmative (Ok)” and moves “Negative” to 10-10.

Source	Date	What 10-4 means
APCO Bulletin, Illinois list	May 1939	”Acknowledgement”
APCO Bulletin, revised list	January 1940	”Acknowledgement”
APCO Bulletin, standard list	1955, printed April 1957	”Message received”
APCO operating manual	February 1967	”Acknowledgement”
APCO Bulletin training column	February 1968	Use it for “I will do it”
APCO Project 14 brevity code	1974	”Affirmative (Ok)”

UPI’s 1986 obituary of Broderick Crawford said the actor “probably was best remembered for barking ‘ten-four, ten-four’ into his police car radio” on Highway Patrol, “the prototype 1950s police television drama.” By the time C.W. McCall’s CB trucker anthem “Convoy” topped the pop chart in 1976, as NPR recounts, 10-4 had been in print for more than 36 years, and the 1940 list had already given 10-14 to “Convoy or escort.”

Four states were using the list before APCO adopted it

The name usually attached to the codes is Charles “Charlie” Hopper, whom Wikipedia calls communications director for Illinois State Police District 10 at Pesotum and credits with the invention. Its citation, a 2006 Law and Order article by James Careless, never mentions him in its archived copy. APCO’s official magazine did run a 2024 article headlined “Charles Hopper and the 10-Codes.”

APCO’s own 1939 Bulletins list a C.L. Hopper of the Illinois State Police at Springfield and name him to the association’s Operating Procedure Committee. In May of that year the Bulletin printed Illinois’ 17 “Operators’ ‘Ten’ Signals,” 10-4 “Acknowledgement” among them. APCO’s 1976 Bulletin says the first ten signals it published, “17 of them,” appeared in 1939.

The 1940 revision came from a standards committee, chaired by T.M.B. Lowe of Springfield, Illinois, making “numerous changes” to “the list that has been in use in Iowa, Illinois, Missouri and Minnesota for the past several years,” with formal action scheduled for March 25, 1940.

APCO’s dates disagree. The 1967 manual says the signals were “originally formulated by Illinois APCO members in 1935,” while a 2009 APCO policy manual dates “the familiar ‘10 Codes’” to 1937. Illinois appears in the 1939 and 1940 lists and the 1967 manual alike, and none of them names an inventor.

Federal doctrine wants it off multi-agency radio

The Department of Homeland Security’s SAFECOM program, in its guide to making the switch, says codes answered two problems: by the 1940s two-way radio “began to overload early single channel radio systems,” and officers wanted privacy on channels that could be monitored.

Agencies also wrote their own lists. SAFECOM’s example is 10-88, which “may mean ‘present phone number’ in one agency and ‘officer needs help’ in another.” The 1940 list had already reserved its “80” series “for assignment by nets for local use,” suggesting 10-88 as a request for an officer’s phone number.

FEMA’s National Incident Management System now states the rule directly: “Using plain language and clear text, not codes, in incident management is a matter of public safety.” SAFECOM notes that NIMS requires it when agencies respond together and strongly encourages it for everyday traffic. Law and Order magazine predicted in 2006 that “soon the only appropriate place for 10-codes will be on CB radios and old reruns of ‘Highway Patrol.’”

Federal doctrine now presses police to drop 10-4 on the radios it was written for, while everyday speech keeps it with the meaning the 1940 list never had: yes. Strictly FYI.