16-line message format

16-line message format, or Basic Message Format, is the NATO standard military radiogram format for the manner in which a paper message form is transcribed through voice, Morse code, or Teletypewriter transmission formats. The overall structure of the message has three parts: HEADING, TEXT, and ENDING. This heading is further divided into procedure, preamble, address, and prefix. Each format line contains pre-defined content. An actual message may have fewer than 16 actual lines, or far more than 16, because some lines are skipped in some delivery methods, and a long message may have a TEXT portion that is longer than 16 lines by itself.
This radiotelegraph message format and transmission procedures have been documented in numerous military standards, going back to at least World War II-era U.S. Army manuals.

Historical Development

U.S. Army Field Manual FM 24-5 Basic Field Manual, Signal Communication
U.S. Army Field Manual FM 24-17 Tactical Communications Center Operations
U.S. Army Field Manual FM-24-19 Radio Operator's Handbook
Messages were formerly prepared on DD Form 173/4, Joint Message Form
DD Form 1753, Master Station Log would be used to record some types of data for the radio station.
DD Form 1765, Incoming Service Message Log would be used to record received messages.
DD Form 1766, Outgoing Service Message Log would be used to record outgoing messages.

Current Definition and Usage

The 16-line format and procedures for transmitting it vary slightly depending on the communications medium, but all variations are designed to be harmonious and the procedures describe how to convert between the formats.

ACP-124
ACP-125 defines the format of messages relayed by voice
ACP-126 defines the format of messages relayed by radio teletype
ACP-127 defines the format of messages relayed by automated tape
Military Affiliate Radio Service

Example Messages

When sent as an ACP-126 message over teletype, a 16-line format radiogram would appear similar to this:

RFHT
DE RFG NR 114
R 151412Z MAR
FM CG FIFTH CORPS
TO CG THIRD INFDIV
WD GRNC
BT
UNCLAS
PLAINDRESS SINGLE ADDRESS
MESSAGES WILL BE TRANSMITTED
OVER TELETIPEWRITER CIRCUITS
AS INDICATED IN THIS EXAMPLE
BT
C WA OVER TELETYPEWRITER
NNNN

Some of the format lines in the above example have been omitted for efficiency. The translation of this abbreviate format follows:

Format Line	Message Text	Explanation
Line 2	RFHT	Station being called, which will receive the message
Line 3	DE RFG NR 114	Sent by radio station having the callsign RFG, station serial number 114
Line 5	R 151412Z MAR	Routine precedence, March 15, 2:12pm UTC in Date-time group format
Line 6	FM CG FIFTH CORPS	The message is from CG FIFTH CORPS
Line 7	TO CG THIRD INFDIV	The message is to CG THIRD INFDIV
Line 10	WD GRNC	Accounting symbol ; word groups have not been counted
Line 11	BT	Section separator between heading and text
Line 12	UNCLAS PLAINDRESS SINGLE ADDRESS MESSAGES WILL BE TRANSMITTED OVER TELETIPEWRITER CIRCUITS AS INDICATED IN THIS EXAMPLE	Message content is unclassified, and the message is sent with a misspelled word, "TELETIPEWRITER" for example purposes.
Line 13	BT	Section separator between text and the ending
Line 15	C WA OVER TELETYPEWRITER	corrects word after "OVER" to "TELETYPEWRITER"
Line 16	NNNN	end-of-message indicator

Example message in four different formats:

Format Line	Telegraph Radiogram	Voice Radiogram	Radioteletype Radiogram	Tape Relay Radiogram	ACP-127 Supl.
1			VZCZC051 UU	VZCZC051 UU	VZCZC051 UU
2		CALLED_CALLSIGN THIS IS CALLING_CALLSIGN MESSAGE NUMBER 051	CALLED_CALLSIGN DE CALLING_CALLSIGN NR 051	RR CALLED_CALLSIGN	RR CALLED_CALLSIGN
3				DE CALLING_CALLSIGN 0051 22/1856Z	DE CALLING_CALLSIGN #0051 1121857
4			TRANSMISSION INSTRUCTIONS/OP. SIGS	ZNR UUUUU TRANSMISSION INSTRUCTIONS/OP. SIGS	ZNR UUUUU TRANSMISSION INSTRUCTIONS/OP. SIGS
5		ROUTINE TIME 221855Z APR 2015	R 221855Z APR 2015	R 221856Z APR 2015	R 221857Z APR 2015
6		FROM ORIGINATION PLAIN LANGUAGE ADDRESS	FM ORIGINATION PLAIN LANGUAGE ADDRESS	FM ORIGINATION PLAIN LANGUAGE ADDRESS	FM ORIGINATION PLAIN LANGUAGE ADDRESS
7		TO TO ADDRESSEE RI AND PLAD	TO TO ADDRESSEE RI AND PLAD	TO TO ADDRESSEE RI AND PLAD	TO TO ADDRESSEE RI AND PLAD
8		INFO INFORMATION ADDRESSEE 1 RI AND PLAD INFORMATION ADDRESSEE 2 RI AND PLAD	INFO INFORMATION ADDRESSEE 1 RI AND PLAD INFORMATION ADDRESSEE 2 RI AND PLAD	INFO INFORMATION ADDRESSEE 1 RI AND PLAD INFORMATION ADDRESSEE 2 RI AND PLAD	INFO INFORMATION ADDRESSEE 1 RI AND PLAD INFORMATION ADDRESSEE 2 RI AND PLAD
9
10		GROUP NO COUNT
11		BREAK	BT	BT	BT
12		UNCLASSIFIED THIS IS A TEST MESSAGE	UNCLAS THIS IS A TEST MESSAGE	UNCLAS THIS IS A TEST MESSAGE	UNCLAS THIS IS A TEST MESSAGE
13		BREAK	BT	BT	BT
14
15					#0051
16		OVER	NNNN	NNNN	NNNN

Historical Development

The concept of the standard message format originated in the wired telegraph services. Each telegraph company likely had its own format, but soon after radio telegraph services began, some elements of the message exchange format were codified in international conventions, and these were then often duplicated in domestic radio communications regulations and in military procedure documentation.
Military organizations independently developed their own procedures, and in addition to differing from the international procedures, they sometimes differed between different branches of the military within the same country.
For example, the publication "Communication Instructions, 1929", from the U.S. Navy Department, includes:

One procedure for messages transmitted "in naval form over nonnaval systems"
One procedure for exchanging messages with commercial radio stations
One procedure for messages transmitted within the Navy
One format for exchanging messages between the Army and Navy, called the "Joint Army and Navy Radiotelegraph Procedure", with the format shown on page 70.