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.
APCO first proposed Morse code brevity codes in the June, 1935 issue of The APCO Bulletin, which were adapted from the procedure symbols of the U.S. Navy.
The development of the APCO Ten Signals 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. An APCO Bulletin of January 1940 lists codes assigned as part of standardization;
The Ten Signals were included in APCO Project Two (1967), "Public Safety Standard Operating Procedures Manual", published as study cards in APCO Project 4 (1973), "Ten Signal Cards", and then revised in APCO Project 14 (1974).
In popular culture
Ten-codes, especially "10-4" (meaning "understood") 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. C. W. McCall's hit song "Convoy" (1975), depicting conversation among CB-communicating truckers, put phrases like 10-4 and what's your twenty? (10-20 for "where are you?") into common use in American English.
The movie Convoy (1978), loosely based on McCall's eponymous song, further entrenched ten-codes in casual conversation.
The title of the police drama television series 10-8: Officers on Duty (which aired on ABC from September 28, 2003 to January 11, 2004), refers to the ten-code for "officer in service and available for calls".
Characters in the outlaw motorcycle club television series Sons of Anarchy (which aired on FX from 2008 to 2014), often use ten-codes, e.g., in Season 4, Episode 12, Jax tells Phil, "I want a 20 on [Wendy]" by end of day.
Replacement with plain language
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.
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, and their use is expressly forbidden in the nationally standardized Incident Command System, as is the use of other codes.
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, As of 2011[update], ten-codes remain in common use in many areas, but are increasingly being phased out in favor of plain language, with nineteen states planning to change to plain English as of the end of 2009[update].
Examples of ten-codes
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.
- The California Highway Patrol uses ten-codes, along with an additional set of eleven- and higher codes.
- 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:
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.
- Telegraph and teletype procedures
- 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.
- Z codes are used by NATO countries in military radio communications.
- Advanced Medical Priority Dispatch System
- Emergency service response codes
- Law enforcement jargon
- List of international common standards
- Military alphabet
- Medical Priority Dispatch System
- Heard on Morning Edition (2009-10-13). "Plain Talk Eases Police Radio Codes Off The Air". NPR. Retrieved 2010-01-23.
- "The APCO Bulletin (June, 1935)" (PDF). Retrieved 7 May 2015.
- James Careless (August 2006). "The End of 10-Codes?". Archived from the original on 2007-09-28. Retrieved 2006-10-11.
- http://www.apcohistory.org/pdf/1940-01-jan_pages1-12.pdf, p.8
- "The Origin of The Ten Code". Retrieved 27 April 2015.
- "APCO Projects". Retrieved 27 April 2015.
- 9-Code, 10-Code. Dispatch Magazine online.
- "Official Ten-Code List Association of Public Communications Officers (APCO)".
- 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" (PDF). SAFECOM program. Department of Homeland Security. Retrieved 2010-01-23.
- Federal Emergency Management Agency. NIMS Frequently Asked Questions. Retrieved on 2014-12-01 from https://www.fema.gov/nims-frequently-asked-questions.
- APCO position statement on Plain Speech in Public Safety Communications
- Mack, Sharon Kiley (January 1, 2010). "Maine police dropping 10-code, switching to plain language". Bangor Daily News. Retrieved October 26, 2011.
- "Association of Public Communications(APCO) 10 Codes".
- Official 10 codes. Retrieved from http://spiffy.ci.uiuc.edu/~kline/Stuff/ten-codes.html.
- "CHP Glossary". California Highway Patrol. Retrieved 2015-11-25.
- 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.
- "Q Codes" (PDF). CB Radio Source. Retrieved 2010-01-23.
- 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 (page http://www.911dispatch.com/info/tencode.html archived on web archive)
- Official Ten-Code List Association of Public Communications Officers (APCO)[dead link]