Ten-codes, officially known as ten signals, are brevity codes used to represent common phrases in voice communication, particularly by law enforcement and in Citizens Band (CB) radio transmissions. The police version of ten-codes is officially known as the APCO Project 14 Aural Brevity Code.
The codes, developed during 1937–1940 and expanded in 1974 by the Association of Public-Safety Communications Officials-International (APCO), allow 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, though these procedures were for communications in Morse code, not voice.
In August 1935, the APCO Bulletin published a recommendation that the organization issue a handbook that described standard operating procedures, including:
- A standard message form for use by all police departments.
- A simple code for service dispatches relating to corrections, repetitions, etc.
- A standard arrangement of the context of messages, (for example, name and description of missing person might be transmitted as follows: Name, age, height, weight, physical characteristics, clothing; if car used, the license, make, description and motor number. This information would actually be transmitted in the text of the message as follows: John Brown 28-5-9-165 medium build brown eyes dark hair dark suit light hat Mich.35 lic.W 2605 Ford S 35 blue red wheels 2345678 may go to Indiana).
- A standard record system for logging the operation of the station.
- Other important records in accordance with the uniform crime reporting system sponsored by the International Association of Chiefs of Police.
The development of the APCO Ten Signals began in 1937 to reduce use of speech on the radio at a time when police radio channels were limited. 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;
In 1954, APCO published an article describing a proposed simplification of the code, based on an analysis conducted by the San Diego Police Department. In the September 1955 issue of the APCO Bulletin, a revision of the Ten-Signals was proposed, and it was later adopted.
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 cultureEdit
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 song, further entrenched ten-codes in casual conversation.
The ten-codes used by the New York Police Department have returned to public attention thanks to the popularity of the television series Blue Bloods. However, it must be noted that the ten-codes used by the NYPD are not the same as those used in the APCO system (see below). For example, in the NYPD system, Code 10-13 means "Officer needs help," whereas in the APCO system "Officer needs help" is Code 10-33.
In the last episode of the anime Kekkai Sensen & Beyond, Leonardo uses the code 10-33 in a message to signal that he is in a situation beyond his control.
Replacement with plain languageEdit
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].
Lists of APCO Ten SignalsEdit
|1937 APCO||1939 First Published Set (17 signals)||1940 (APCO Standards Committee)||1955 (National Operating Procedure Committee)||APCO Project 2 (1967)||APCO Project 4 (1973)||APCO Project 14 (1974)||Clear Speech
(plain language to replace Ten Codes)
|Phrase Word Brevity Code (c. 1979)|
|Procedure and Officer Details|
|10-1||Receiving poorly.||Receiving poorly||Unable to copy - change location||Signal Weak||Unable to copy - change location||Unreadable|
|10-2||Receiving well.||Receiving well||Signals good||Signal Good||—|
|10-3||Stop transmitting.||Disregard last information||Stop transmitting||Stop Transmitting||Stop transmitting|
|10-4||Acknowledgement.||Message received||Acknowledgement||Affirmative (Ok)||Roger||Roger/Affirmative|
|10-6||Busy.||Busy, stand by||Busy -Stand by unless urgent||Busy||Busy|
|10-7||Out of service.||Out of service||Out of service (Give location and/or telephone number)||Out of Service||Out at ...||Out of Service|
|10-7 A||—||Not Available|
|10-7 B||—||Off Radio|
|10-8||In service.||In service||In service||In Service||Clear||In Service|
|10-9||Repeat, conditions bad.||Repeat||Repeat||Say Again||Say again||Say Again|
|10-10||Out of service--subject to call.||On minor detail, subject to call||Fight in progress||Negative||—|
|10-11||Dispatching too rapidly.||Stay in service||Dog Case||... On Duty||—||On Radio|
|10-12||Officials or visitors present.||Visitors or officials present||Stand by (stop)||Stand By (Stop)||Stand by||Stand By|
|10-13||Advise weather and road conditions.||Weather and road conditions||Weather and road report||Existing Conditions||Weather report/road report|
|10-14||Convoy or escort.||Convoy or escort||Report of prowler||Message/Information||—||Prepare to Copy|
|10-15||We have prisoner in custody.||We have prisoner in custody||Civil disturbance||Message Delivered||Disturbance|
|10-16||Pick up prisoner at ...||Pick up prisoner at||Domestic trouble||Reply to Message||—|
|10-17||Pick up papers at ...||Pick up papers at||Meet complainant||Enroute||—||Responding|
|10-18||Complete present assignment as quickly as possible.||Anything for us?||Complete assignment quickly||Urgent||Urgent||Priority|
|10-19||Return to your station.||Nothing for you||Return to ...||(In) Contact||Return to ...|
|10-20||What is your location?||Location||Location||Location||Location|
|10-21||Call this station by telephone.||Call ... by phone||Call ... by telephone||Call (...) by Phone||Call ...||Telephone|
|10-22||Take no further action last information.||Report in person to ...||Disregard||Disregard||Disregard||Disregard|
|10-23||Stand by until no interference.||Arrived at scene||Arrived at scene||Arrived at Scene||On scene|
|10-24||Trouble at station--unwelcome visitors--all units vicinity report at once.||Finished with last assignment||Assignment completed||Assignment Completed||—||Available|
|10-25||Do you have contact with...?||Operator or officer on duty?||Report in person to (meet) ...||Report to (Meet) ...||Meet ... or contact ...|
|10-26||Can you obtain automobile registration information?||Holding subject, rush reply||Detaining subject, expedite||Estimated Arrival Time||Detaining subject, expedite|
|10-27||Any answer our number...?||Request driver's license information||Drivers license information||License/Permit Information||Drivers license information on ...|
|10-28||Check full registration information.||Request full registration information||Vehicle registration information||Ownership Information||Registration information on ...|
|10-29||Check for wanted.||Check record for wanted||Check records for wanted.||Records Check||Check for wanted on ...|
|Emergency or Unusual|
|10-30||Does not conform to rules and regulations.||Does not conform to rules and regulations||Illegal use of radio||Danger/Caution||—||Use Caution|
|10-31||Is lie detector available?||Emergency basis, all squads, 10-11||Crime in progress||Pick Up||—|
|10-32||Is drunkometer available?||Chase, all squads stand by||Man with gun||... Units Needed (Specify)||—|
|10-33||Emergency traffic at this station--clear?||Emergency traffic this station||EMERGENCY||Help Me Quick||—||Help Officer|
|10-34||Clear for local dispatch?||Trouble at station, assistance needed||Riot||Time||—|
|10-35||Confidential information.||Major crime, blocade||Major crime alert||—Reserved—||—|
|10-36||Correct time?||—||Correct time||—Reserved—||—|
|10-37||Operator on duty?||No rush||Investigate suspicious vehicle||—Reserved—||—|
|10-38||Station report--satisfactory.||Hurry, but do not use red light or siren||Stopping suspicious vehicle (Give station complete description before stopping).||—Reserved—||Traffic stop on ...|
|10-39||Your Nr...delivered to addressee.||Use red light and siren||Urgent-Use light and siren||—Reserved—||—|
|General Use||Private Use|
|10-40||Advise if Officer...available for radio call.||Notification||Silent run - No light or siren||—|
|10-41||Tune to ... kcs. for test with mobile unit or emergency service.||Car change at ...||Beginning tour of duty||—|
|10-42||—||Crew change at ...||Ending tour of duty||Off duty|
|10-43||—||Take school crossing||Information||—|
|10-44||—||—||Request permission to leave patrol ... for ...||Request for ...|
|10-45||—||—||Animal carcass in ... lane at ...||—|
|10-46||—||—||Assist motorist||Assist motorist|
|10-47||—||—||Emergency road repairs needed||—|
|10-48||—||—||Traffic standard needs repairs||—|
|10-49||—||Hourly report mark||Traffic light out||East bound green light out (etc.)|
|Accident and Vehicle Handling|
|10-50||—||Auto accident, property damage only||Accident -- F, PI, PD||Traffic (F, PD)
|10-51||—||Auto accident, wrecker sent||Wrecker needed||—|
|10-52||—||Auto accident, personal injuries, ambulance sent||Ambulance needed||—|
|10-53||—||Auto accident, fatal||Road blocked||—|
|10-54||—||—||Livestock on highway||—|
|10-55||—||Drunken driver||Intoxicated driver||—|
|10-56||—||—||Intoxicated pedestrian||Drunk pedestrian|
|10-57||—||—||Hit and run -- F, PI, PD||—|
|10-58||—||Is wrecker on the way?||Direct traffic||—|
|10-59||—||Is ambulance on the way?||Convoy or escort||—|
|Net Message Handling|
|10-60||What is next item (message) number?||What is your next message number?||Squad in vicinity||—|
|10-61||Stand by for CW traffic on ... kcs.||CW traffic||Personnel in area.||—|
|10-62||Unable to copy phone--use CW.||Any answer our Nr. ...||Reply to message||—|
|10-63||Net directed.||Time||Prepare to make written copy||Prepare to copy|
|10-64||Net free.||—||Message for local delivery||—|
|10-65||Clear for item (message) assignment?||Clear for message assignment||Net message assignment||—|
|10-66||Clear for cancellation?||Clear for cancellation||Message cancellation||—|
|10-67||Stations...carry this item (message).||Clear for net message||Clear to read net message||—|
|10-68||Repeat dispatch.||—||Dispatch information||—|
|10-69||Have you dispatched...?||—||Message received||—|
|10-70||Net message (State net traffic).||Fire, phone alarm||Fire alarm||Fire|
|10-71||Proceed with traffic in sequence (busy here).||Box alarm||Advise nature of fire (size, type, and contents of building)||—|
|10-72||—||Second alarm||Report progress on fire||—|
|10-73||—||Third alarm||Smoke report||—|
|10-75||—||Fifth alarm||In contact with||—|
|10-76||—||Fire equipment needed||En Route||En route ...|
|10-77||—||Fire, grass||ETA (Estimated Time of Arrival)||ETA (Estimated time of arrival)|
|10-78||—||Set up command post||Need assistance||Request Assistance|
|10-79||—||Report progress on fire||Notify coroner||Notify coroner (to be done by phone whenever possible)|
|The 80 series is reserved for assignment by nets for local use.||Personal Favors||—|
|10-80||... tower lights at this station burned out.||—||—||Chase|
|10-81||Officer Nr. ... will be at your station ...||—||—||—|
|10-82||Reserve room with bath at hotel for officer Nr. ...||Reserve hotel room||Reserve lodging||—|
|10-83||Have officer Nr. ... call this station by telephone.||—||—||—|
|10-84||Advise telephone Nr. ... your city that officer Nr. ... will not return this date.||—||If meeting ... advise ETA||—|
|10-85||Officer ... left this station for ... (Jefferson City) (Des Moines) at ...||—||Will be late||—|
|10-86||Officer ... left this station for ... at ...||—||—||—|
|10-87||Officer Nr. ... will be in ... if officer Nr. ... will be in.||—||Pick up checks for distribution||—|
|10-88||What phone number shall we call to make station to station call to officer Nr. ...?||Advise phone number for station to station call||Advise present telephone number of ...||—|
|10-89||Request radio service man be sent to this station...||Radio transmission||—||Bomb threat|
|10-90||Radio service man will be at your station ....||Transmit on alternate frequency||Bank alarm||Alarm (type of alarm)|
|10-91||Prepare for inspection (date) ... (time) ...||—||Unnecessary use of radio||Pick up prisoner|
|10-92||Your quality poor--transmitter apparently out of adjustment.||—||—||Parking complaint|
|10-93||Frequencies to be checked this date.||Frequency check||Blockade||—|
|10-94||Test--no modulation--for frequency check.||Give me a test||Drag racing||—|
|10-95||Test intermittently with normal modulation for ...||—||—||Prisoner in custody|
|10-96||Test continuously with tone modulation for ...||—||Mental subject||—|
|10-97||—||—||—||Check traffic signal|
|10-98||—||—||Prison or jail break||Prison/jail break|
|10-99||—||—||Records indicate wanted or stolen||Wanted/stolen|
Clear Speech ProcedureEdit
In 1971, the Public Safety Department of Lakewood, Colorado published a study comparing the APCO Ten-code with the proposed Clear Speech procedure. The study used standards for judgment of both communications procedures based on The Public Safety Communications Standard Operating Procedure Manual, 1970 edition, published by APCO.
|10-1||Unable to copy - change location|
|10-13||Weather report/road report|
|10-26||Detaining subject, expedite|
|10-27||Drivers License information on...|
|10-28||Registration information on...|
|10-29||Check for wanted on...|
|10-38||Traffic stop on...|
|10-49||East bound green light out (etc.)|
|10-63||Prepare to copy|
|10-77||ETA (Estimated time of arrival)|
|10-79||Notify coroner (to be done by phone whenever possible)|
|10-90||Alarm (type of alarm)|
|10-91||Pick up prisoner|
|10-95||Prisoner in custody|
|10-97||Check traffic signal|
- Roger—To be used as acknowledgement.
- AFFIRMATIVE—To be used when "yes" is needed
- HELP—To be used when in danger and urgent assistance is needed.
|CODE ONE||Informs all units to STANDBY - STOP TRANSMITTING. Do not transmit, except for emergency messages, while Code 1 is in effect. Dispatch shall announce, "Clear Code 1," when the condition is secured.|
|CODE TWO||Indicates an "urgent" call short of an "emergency" situation. A Code 2 call has priority over all other police activities except "emergencies".
Proceed directly to Code 2 calls as quickly as is consistent with safety. Agents may, in exceptional cases, use their emergency equipment (both visual and audible to comply with state law) to transverse an otherwise clear intersection against a red traffic control device. Once clear of the intersection - turn off the emergency equipment.
|CODE THREE||Indicates an EMERGENCY call. Red lights and siren are authorized. Proceed as quickly as possible with due regard for safety, and in compliance with the laws governing emergency vehicles.|
|CODE FOUR||Used to indicate that sufficient units have responded to a location, or that assistance is not needed, or is no longer needed.|
|CODE FIVE||Used when Wanted/Records checks are requested by an agent to alert the agent of a wanted felon, a person known to be dangerous or a person known to be mentally unstable.
A backup unit shall be dispatched Code 2 on all Code 5's.
Personnel will NOT proceed with Code 5 details until the receiving unit requests same. The unit receiving a Code 5 will request the details when he is in a safe position to do so, which might not be until his backup arrives.
|CODE SIX||When an agent is dispatched to a traffic accident, and the dispatcher states, "Code 6," the agent will advise the drivers involved to proceed to the situation to file their reports. This will only be done if there are no injuries, no unusual circumstance and the vehicles are safely operable. Driver Exchange Forms will be completed at the scene to include the C. R. number.|
|CODE SEVEN||Indicates "out of service - personal."|
|CODE EIGHT||Assist a fire department.|
Phrase Word Brevity CodeEdit
About 1979, APCO created the Phrase Word Brevity Code as a direct replacement for the Ten-code.
|Phrase Word||English Meaning||APCO TEN CODE|
|Use Caution||Caution: dangerous condition is suspected to exist.||10-0|
|Unreadable||Radio signal is too weak to receive.||10-1|
|Out of Service||Unit, vehicle or person is not working||10-7|
|In Service||Unit, vehicle or person is working but not necessarily "available" or "on radio."||10-8|
|Available||Unit is in service ready to accept assignment, not necessarily by radio.||10-24|
|Not Available||Unit cannot accept another assignment, but may be "on radio."||10-7A|
|Prepare to Copy||Dispatcher is about to give lengthy message.||10-14|
|Go Ahead||You have been given clearance to transmit your message.||- -|
|Roger (Received)||Message received and understood.||10-4|
|Say Again (Repeat)||Repeat your message.||10-9|
|Stand By||Stop transmitting and wait for further instructions.||10-12|
|Disregard (Recall)||Cancel your present assignment.||10-22|
|Off Radio||Unit is not capable of being contacted by radio, but may be "available."||10-7B|
|On Radio||Unit is capable of being contacted by radio, but not necessarily "available."||10-11|
|Responding||Unit is en route to assigned location.||10-17|
|Under Control||Situation is under control when no further assistance is anticipated.||- -|
|Telephone (Tel. # or person)||Call by telephone specified number or person.||10-21|
|Priority||When transmitted, means that the following transmission must have immediate attention.||- -|
|In Pursuit||Unit is chasing a vehicle and requires assistance from other units.||- -|
|Traffic Stop||Unit is going to stop a motorist.||- -|
|Help Officer||Help me quick (emergency).||10-33|
ICS Clear TextEdit
In 1980, the National Incident Management System published a document, ICS Clear Text Guide, which was another attempt to create a replacement for Ten-codes. The list of code words was republished in the 1990 Montana Mutual Aid and Common Frequencies document.
|Unreadable||Used when signal received is not clear. In most cases, try to add the specific trouble. Example: "Unreadable, background noise."|
|Loud and Clear||Self-explanatory|
|Copy, Copies||Used to acknowledge message received. Unit radio identifier must also be used. Example: "Engine 2675, copies."|
|Respond, Responding||Used during dispatch - proceed to or proceeding to an incident. Example: "Engine 5176, respond ..." or "St. Helena, Engine 1375 responding."|
|Enroute||Normally used by administrative or staff personnel to designate destinations. Enroute is NOT a substitute for responding. Example: "Redding, Chief 2400 enroute RO II."|
|In-quarters, with Station Name or Number||Used to indicate that a units is in a station. Example: "Morgan Hill, Engine 4577 in-quarters, Sunol."|
|Uncovered||Indicates a unit is no in-service, because there are no personnel to operate it.|
|Out-Of-Service||Indicates a unit is mechanically out of service. Example: "Aburn, transport 2341, out-of-service." Note, when repairs have been completed the following phrase should be used: "Aburn transport 2341, back in-service, available."|
|In-Service||This means that the unit is operating, not in response to a dispatch. Example: "Fortuna, Engine 1283, in-service, fire prevention inspections."|
|Return to||Normally used by communications center to direct units that are available to a station or other location.|
|What is your Location?||Self-explanatory|
|Call ____ by Phone||Self-explanatory|
|Disregard Last Message||Self-explanatory|
|Vehicle Registration Check||Self-explanatory|
|Is ____ Available for a Phone Call?||Self-explanatory|
|At Scene||Used when Units arrive at the scene of an incident. Example: "Perris, Engine 6183, at scene."|
|Available at Residence||Used by administrative or staff personnel to indicate they are available and on-call at their residence.|
|Can Handle||Used with the amount of equipment needed to handle the incident. Example: "Susanville Battalion 2212, can handle with units not at scene."|
|Report on Conditions||Self-explanatory|
|Fire under Control||Self-explanatory|
|Emergency Traffic Only||Radio users will confine all radio transmissions to an emergency in progress or a new incident. Radio traffic which includes status information such as responding, reports on conditions, at scene and available will be authorized during this period.|
|Emergency Traffic||Term used to gain control of radio frequency to report and emergency. All other radio users will refrain from using that frequency until cleared for use by the communications center.|
|Resume Normal Traffic||Self-explanatory|
- 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. 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 American public was made aware of these California Penal Code references as a result of the TV series Adam-12, which used them habitually in radio communications and in the main title of the show. The best-known include:
- "187": Homicide
- "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)
- 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".
- 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, and provides codes standardized by meaning in all languages – essential for international shortwave radio communications.
- Z codes are used for military radio communications NATO countries, and like Q codes are standardized across languages.
- "APCO Brevity Code to be "Voluntary Standard" In Florida Communications Plan" (PDF).
- Heard on Morning Edition (2009-10-13). "Plain Talk Eases Police Radio Codes Off The Air". NPR. Archived from the original on 2009-10-17. Retrieved 2010-01-23.
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- "Special APCO Bulletin" (PDF). August 1935.
- "APCO Project Series" (PDF).
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- "Codes And Signals - More Discussion On The Radio Code Problems".
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- The End of the Ten-Code? Archived 2009-07-22 at the Wayback Machine.. 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. Archived from the original (PDF) on 2010-05-27. Retrieved 2010-01-23.
- Federal Emergency Management Agency. NIMS Frequently Asked Questions. Retrieved on 2014-12-01 from "Archived copy". Archived from the original on 2014-12-23. Retrieved 2014-12-02..
- APCO position statement on Plain Speech in Public Safety Communications Archived 2012-05-09 at the Wayback Machine.
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- "Happy Birthday" (PDF).
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- "A NATIONAL TRAINING MANUAL AND PROCEDURAL GUIDE FOR POLICE AND PUBLIC SAFETY RADIO COMMUNICATIONS PERSONNEL".
- "Public Safety Communication Aural Brevity Code" (PDF).
- "OFFICIAL TEN SIGNAL LIST" (PDF). Associated Public-Safety Communications Officers, Inc.
- "Ten Code Versus Clear Speech Communication" (PDF). Archived from the original on 2006-03-13.
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- Ten-codes require transmission of three prefix characters "10-" and two numbers, so five characters, on top of which digits and punctuation are all long sequences in Morse (5–6 dits or dahs). Letters are all short sequences in Morse (1–4 dits or dahs), so the prefix "Q" and two letters is fewer characters and shorter code sequences.
- APCO Bulletin—The APCO Bulletin, January 1940. The first official publication showing the 10-codes (see page 8).
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