Ten-code

"10-1" redirects here. For the FIFA World Cup match, see Hungary v El Salvador (1982 FIFA World Cup).

"10-20" redirects here. For the EEG electrode placement system, see 10–20 system (EEG).

Ten-codes, officially known as ten signals, are brevity codes used to represent common phrases in voice communication, particularly by US public safety officials 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 in 2006, due to the lack of standardization, the U.S. federal government recommended they be discontinued in favor of everyday language.

History

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:

1. A standard message form for use by all police departments.
2. A simple code for service dispatches relating to corrections, repetitions, etc.
3. 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).
4. A standard record system for logging the operation of the station.
5. 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 standardisation.

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 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 song, further entrenched ten-codes in casual conversation, as does the movie Smokey and the Bandit.

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, the ten-codes used by the NYPD are not the same as those used in the APCO system. 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.

The New Zealand reality television show Ten 7 Aotearoa (formerly Police Ten 7) takes its name from the New Zealand Police ten-code 10-7, which means "Unit has arrived at job".

The syndicated internet radio countdown program "What's your Twenty" is named after the code for location.

The Chicago Police Department uses the radio code '10-1', which means an officer needs urgent help right away. The Chicago P.D. (TV series) TV show also uses '10-1' as well.

Police officer retirement

Often when an officer retires, a call to dispatch is made. The officer gives a 10-7 code (Out of service) and then a 10-42 code (ending tour of duty).

Signals by era

Signal			APCO Meaning
	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 (c. 1971) (plain language to replace Ten Codes)	Phrase Word Brevity Code (c. 1979)
Procedure and Officer Details
10-MAX		AWOL	-	-	-		Corrupt Officer	Officer not following orders	Insubord­ination/​Rogue Officer
									Go Ahead
									Under Control
									In Pursuit
									Traffic Stop
10-0			—	—	—		—	Use caution
10-1			Receiving poorly.		Unable to copy - change location		Signal Weak	Unable to copy - change location	Unreadable
10-2			Receiving well.		Signals good		Signal Good	—
10-3			Stop transmitting.	Disregard last information	Stop transmitting		Stop Transmitting
10-4			Acknowl­edgement.	Message received	Acknowl­edgement		Affirmative (Ok)	Roger	Roger/​Affirmative
10-5			Relay.				Relay (To)	Relay
10-6			Busy.	Busy, stand by	Busy -Stand by unless urgent		Busy	Busy
10-7			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	Clear	In Service
10-9			Repeat, conditions bad.	Repeat			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.		Civil disturbance		Message Delivered	Disturbance
10-16			Pick up prisoner at ...		Domestic trouble		Reply to Message	—
10-17			Pick up papers at ...		Meet complainant		Enroute		Responding
10-17 A			—	—	—		—	Theft
10-17 B								Vandalism
10-17 C								Shoplifting
10-18			Complete present assignment as quickly as possible.	Anything for us?	Complete assignment quickly		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 Phone	Call ...	Telephone
10-22			Take no further action last information.	Report in person to ...	Disregard		Disregard
10-23			Stand by until no interference.	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.		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	—
10-31 A			—	—	—		—	Burglary
10-31 B								Robbery
10-31 C								Homicide
10-31 D								Kidnapping
10-31 E								Shooting
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, blockade	Major crime alert		—Reserved—
10-36			Correct time?	—	Correct time
10-37			Operator on duty?	No rush	Investigate suspicious vehicle
10-38			Station report—satisfactory.	Hurry, but do not use red light or siren	Stopping suspicious vehicle (Give station complete description before stopping).			Traffic stop on ...
10-39			Your Nr...delivered to addressee.	Use red light and siren	Urgent-Use light and siren			—
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) Traffic Hit and run Injury No injury reported Unknown Private property, location
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
Fire
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
10-74				Fourth alarm	Negative			Negative
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.		Meet the officer at ______.
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
Technical
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

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 stated in 2012 that plain speech communications over public safety radio systems is preferred over the traditional 10-Codes and dispatch signals. Nineteen states had changed to plain English by the end of 2009. As of 2011, ten-codes remained in common use in many areas, but were increasingly being phased out in favor of plain language.

Clear Speech Procedure

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. According to the report, the Clear Speech Procedure was to replace ten-codes in all Public Safety Department radio communication since January 28, 1972.

Replacement of ten-codes with the Clear Speech Procedure (Public Safety Department, Lakewood, Colorado, 1972)

Was (Ten-Code)	Now (Procedure Word)
10-0	Use Caution
10-1	Unable to copy – change location
10-3	Stop transmitting
10-4	Roger
10-5	Relay
10-6	Busy
10-7	Out at...
10-8	Clear
10-9	Say again
10-12	Stand by
10-13	Weather report/road report
10-15	Disturbance
10-17A	Theft
10-17B	Vandalism
10-17C	Shoplifting
10-18	Urgent
10-19	Return to...
10-20	Location
10-21	Call...
10-22	Disregard
10-23	On scene
10-25	Meet...or contact...
10-26	Detaining subject, expedite
10-27	Drivers License information on...
10-28	Registration information on...
10-29	Check for wanted on...
10-31A	Burglary
10-31B	Robbery
10-31C	Homicide
10-31D	Kidnapping
10-31E	Shooting
10-38	Traffic stop on...
10-42	Off duty
10-44	Request for...
10-46	Assist motorist
10-49	East bound green light out (etc.)
10-50	Traffic K=Fatal PD=Property Damage Hit and run Injury No injury reported Unknown Private property, location
10-56	Drunk pedestrian
10-63	Prepare to copy
10-70	Fire
10-74	Negative
10-76	En route...
10-77	ETA (Estimated time of arrival)
10-78	Request assistance
10-79	Notify coroner (to be done by phone whenever possible)
10-80	Chase
10-89	Bomb threat
10-90	Alarm (type of alarm)
10-91	Pick up prisoner
10-92	Parking complaint
10-95	Prisoner in custody
10-97	Check traffic signal
10-98	Prison/jail break
10-99	Wanted/stolen
	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.

Clear Speech status codes (Public Safety Department, Lakewood, Colorado, 1972)

Code	Meaning
CODE ZERO	Echo response
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 traverse 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 some. 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 station 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 Code

About 1979, APCO created the Phrase Word Brevity Code as a direct replacement for the Ten-code.

Phrase Word Brevity 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	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
Affirmative	Yes.	10-4

ICS Clear Text

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.

ICS Clear Text Guide

Procedure Word	Meaning
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
Stop Transmitting
Copy, Copies	Used to acknowledge message received. Unit radio identifier must also be used. Example: "Engine 2675, copies."
Affirmative	Yes
Negative	No
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 unit is in a station. Example: "Morgan Hill, Engine 4577 in-quarters, Sunol."
Uncovered	Indicates a unit is not 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."
Repeat	Self-explanatory
Weather
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
Disregard Last Message
Stand-By
Vehicle Registration Check
Is ____ Available for a Phone Call?
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."
Burning Operations	Self-explanatory
Report on Conditions
Fire under Control
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 not be authorized during this period.
Emergency Traffic	Term used to gain control of radio frequency to report an emergency. All other radio users will refrain from using that frequency until cleared for use by the communications center.
Resume Normal Traffic	Self-explanatory

Related codes

Main article: Police code

Brevity codes other than the APCO 10-code are frequently used, and include several types:

• 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
    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)
• 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.

See also

• Advanced Medical Priority Dispatch System
• Emergency service response codes
• List of CB slang
• List of international common standards
• Medical Priority Dispatch System
• NATO phonetic alphabet
• Radiotelephony procedure
  • Procedure word
• Spelling alphabet

External links

• The APCO Bulletin, January 1940 – The first official publication showing the 10-codes.
• Official Ten-Code List Association of Public Communications Officers (APCO)