The codes, developed in 1937 and expanded in 1974 by the Association of Public-Safety Communications Officials-International (APCO), allow for brevity and standardization of message traffic. They have historically been widely used by law enforcement officers in North America but due to the lack of standardization, in 2006 the U.S. federal government recommended they be discontinued in favor of everyday language.
The development of the 10-codes began in 1937, when police radio channels were limited to reduce use of speech on the radio. Credit for inventing the codes goes to Charles "Charlie" Hopper, communications director for the Illinois State Police, District 10 in Pesotum, Illinois. Hopper had been involved in radio for years and realized there was a need to abbreviate transmissions on State Police bands. Experienced radio operators knew the first syllable of a transmission was frequently not understood because of quirks in early electronics technology. Radios in the 1930s were based on vacuum tubes powered by a small motor-generator called a dynamotor. The dynamotor took from 1/10 to 1/4 of a second to "spin up" to full power. Police officers were trained to push the microphone button, then pause briefly before speaking; however, sometimes they would forget to wait. Preceding each code with "ten-" gave the radio transmitter time to reach full power.
Ten-codes, especially "ten-four", first reached public recognition in the mid- to late-1950s through the popular television series Highway Patrol, with Broderick Crawford. Crawford would reach into his patrol car to use the microphone to answer a call and precede his response with "10-4". Ten-codes were adapted for use by CB radio enthusiasts. The 1975 hit song "Convoy" by C. W. McCall depicting conversation among CB-communicating truckers put phrases like 10-4 meaning "understood" and what's your twenty? (10-20) for "where are you?" into common use in American English. A 1978 movie Convoy, loosely based on the song, further entrenched ten-codes in casual conversation.
Replacement with plain language
As of 2011[update], ten-codes remain in common use, but have been phased out in some areas in favor of plain language. Nineteen states were planning to change to plain English as of the end of 2009[update].
Codes are often used inefficiently. For instance, an exchange that could be "1 Mike 1, 10-20?" "First and Main" might be more like "1 Mike 1, what's your 10-20?" "My 20 is First and Main"—it would be more efficient to simply ask, "1 Mike 1, where are you?" "I'm at First and Main." On the other hand, there are times when the use of codes is appropriate, even if less efficient than speaking "in the clear". For instance, using discreet codes for sexual assault, homicide, suicide, and other such situations can prevent the victim and family from having to hear the description being broadcast to all within earshot. Even when the meaning is known, it is less of an emotional jolt to hear a set of numbers being rattled off than to hear plain-speech terms for the trauma.
While ten-codes were intended to be a terse, concise, and standardized system, the proliferation of different meanings can render them useless in situations when officers from different agencies and jurisdictions need to communicate. For that reason, their use is expressly forbidden in the nationally standardized Incident Command System, as is the use of other codes.
In the fall of 2005, responding to inter-organizational communication problems during the rescue operations after Hurricane Katrina, the United States Federal Emergency Management Agency (FEMA) discouraged the use of ten-codes and other codes due to their wide variation in meaning. The Department of Homeland Security's SAFECOM program, established in response to communication problems experienced during the September 11 attacks also advises local agencies on how and why to transition to plain language. The New Orleans Police Department continued using 10-codes as of 2010[update]. One solution to the inter-jurisdictional problem would be to establish a universal standard for the most common 10-codes and disallow any others.
Examples of ten-codes
While APCO International's current position states that plain speech communications over public safety radio systems is preferred over the traditional 10-Codes and dispatch signals, an APCO Bulletin of January 1940 lists codes assigned as part of standardization; in 1973, APCO Project 14 provided a core list of codes from 10-1 to 10-39 with "optional" codes above 10-39.
Some examples of the codes are:
- 10-1 Bad reception
- 10-4 I acknowledge 
- 10-9 Say again
- 10-20 Advise to location
- 10-33 Emergency - all units stand by
- 10-36 Correct time
Many additional codes have been added by individual local or regional first-response agencies; these are not standard across jurisdictions and may be problematic if multiple organizations must respond to the same incident.
California Penal Code sections were in use by the Los Angeles Police Department as early as the 1940s, and these Hundred Code numbers are still used today instead of the corresponding ten-code. The best-known include:
- "187": Homicide
Further information: 187 (slang)
- "211": Robbery
- "415": Disturbance
- "417": Person with a weapon
- "502": Intoxicated Driver
- "5150": mentally disturbed person (actually a reference to the California Welfare and Institutions Code)
Generally these are given as two sets of numbers—"One Eighty-Seven" or "Fifty-One Fifty"—with a few exceptions such as "459"—Burglary, which is given as "Four-Five-Nine".
The New York Fire Department uses its own ten-code system
The New Zealand Fire Service uses a system of "K-codes" to pass fire appliance availability statuses as well as operational messages. For example, "K1" means "proceeding to incident", while "K99" means "Structure fire, well involved". The New Zealand Police also use some K-codes, with completely unrelated meanings to those used by NZFS; Police code "K1" means "no further police action required". The New Zealand Coordinated Incident Management System uses the same common language (although slightly different from the Incident Command System) to avoid inter-agency confusion.
Q code and prosigns for Morse code are used in amateur radio, aviation and marine radio. They provide specific abbreviations for concepts related to aviation, shipping, RTTY, radiotelegraph and amateur radio. In radiotelegraph operation, a Q code is often shorter (as ten-codes require transmission of three prefix characters: 1, 0, hyphen) and provides standardization of codes, essential in international and shortwave communications.
- Emergency service response codes
- Medical Priority Dispatch System
- Advanced Medical Priority Dispatch System
- 10-8: Officers on Duty, a short-lived ABC police drama
- List of CB slang
- List of international common standards
- Q code
- Voice procedure
- Z code
- Heard on Morning Edition (2009-10-13). "Plain Talk Eases Police Radio Codes Off The Air". NPR. Retrieved 2010-01-23.
- James Careless (August 2006). "The End of 10-Codes?". Archived from the original on 2007-09-28. Retrieved 2006-10-11.
- Mack, Sharon Kiley (January 1, 2010). "Maine police dropping 10-code, switching to plain language". Bangor Daily News. Retrieved October 26, 2011.
- Federal Emergency Management Agency. NIMS Frequently Asked Questions. Retrieved on 2014-12-01 from https://www.fema.gov/nims-frequently-asked-questions.
- The End of the Ten-Code?. Tim Dees, Officer.com, 9 November 2005
- 10-4 no more?. Megan Scott, asap (AP), 23 November 2005
- "Plain Language Guide". SAFECOM program. Department of Homeland Security. Retrieved 2010-01-23.
- New Orleans Police Department and Metro Area Live Audio Feed. Retrieved 25 February 2010.
- Feds working to end use of 10-codes
- APCO position statement on Plain Speech in Public Safety Communications
-  APCO standard 10 codes
- "Official Ten-Code List Association of Public Communications Officers (APCO)".
- "Association of Public Communications(APCO) 10 Codes".
- Official 10 codes. Retrieved from http://spiffy.ci.uiuc.edu/~kline/Stuff/ten-codes.html.
- F.D.N.Y. Radio Codes The Unofficial Home Page of FDNY.
- "K-Codes", Tokoroa Volunteer Fire Brigade
- "Police K code". Radio Wiki. 2012-06-07.
- "CHP Glossary". California Highway Patrol. Retrieved 2010-06-21.
- "Q Codes". CB Radio Source. Retrieved 2010-01-23.}
- Los Angeles Sheriff's Department ten code list[dead link]
- New York City Police Department radio signal codes
- FDNY 10 codes
- APCO Bulletin—The APCO Bulletin, January 1940. The first official publication showing the 10-codes (see page 8).
- Association of Public Communications – Official ten code list