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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 during its pop culture explosion in the late 1970s. 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.
10-4 does not mean "yes" per se, but rather "understood", "acknowledged", or "okay."
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 where people 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.
Q: Our 911 center, which receives and dispatches emergency and non-emergency calls, has told us that we may not use 10-codes at all. I gather we must use plain language when using NIMS ICS. Am I correct? A: Yes, when engaged using ICS, plain language is required. The value of using 10-codes for simplicity and speed is lost when others are confused, as may occur in a multi-jurisdiction/multi-agency response: the codes used in one jurisdiction or agency may not be the same in another. It is important that responders and incident managers use common terminology to prevent misunderstanding. While plain English is not required internally, it is encouraged over 10-codes to promote familiarity with emergency operational procedures.
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. One solution to the inter-jurisdictional problem would be to establish a universal standard for the most common 10-codes and disallow any others.
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:
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:
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. The New Zealand Police also use some K-codes, with completely unrelated meanings to those used by NZFS. For example, the NZFS code "K1" means "proceeding to incident", while the Police code "K1" means "no further police action required". The New Zealand Coordinated Incident Management System uses the same common language as 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.