Procedural signs or prosigns are shorthand signals used in Morse code telegraphy, for the purpose of simplifying and standardizing procedural protocols for landline and radio communication. The procedural signs are distinct from conventional Morse code abbreviations, which consist mainly of brevity codes that convey messages to other parties with greater speed and accuracy. However, some codes are used both as prosigns and as single letters or punctuation marks, and for those, the distinction between a prosign and abbreviation is ambiguous, even in context.

In the broader sense prosigns are just standardised parts of short form radio protocol, and can include any abbreviation. Examples would be K for "okay, heard you, continue" or R for "message, received".^[1][2] In a more restricted sense, "prosign" refers to something analogous to the nonprinting control characters in teleprinter and computer character sets, such as Baudot and ASCII. Different from abbreviations, those are universally recognizable across language barriers as distinct and well-defined symbols.

At the coding level, prosigns admit any form the Morse code can take, unlike abbreviations which have to be sent as a sequence of individual letters, like ordinary text. On the other hand, most prosigns codes are much longer than typical codes for letters and numbers. They are individual and indivisible code points within the broader Morse code, fully at par with basic letters and numbers.

The development of prosigns began in the 1860s for wired telegraphy. Since telegraphy preceded voice communications by several decades, many of the much older Morse prosigns have acquired precisely equivalent prowords for use in more recent voice protocols.

Not all prosigns used by telegraphers are standard: There are regional and community-specific variations of the coding convention used in certain radio networks to manage transmission and formatting of messages, and many unofficial prosign conventions exist; some of which might be redundant or ambiguous. One typical example of something which is not an officially recognized prosign, but is yet fairly often used in Europe, is one or two freely timed dits at the end of a message, I  I or   ▄ ▄    ▄ ▄ ; it is equivalent to the proword OUT, meaning "I'm done; go ahead". However the official prosign with the same meaning is AR, or   ▄ ▄▄▄ ▄ ▄▄▄ ▄ , which takes a little longer to send.^[3][2]

Even though represented as strings of letters, prosigns are rendered without the intercharacter commas or pauses that would occur between the letters shown, if the representation were (mistakenly) sent as a sequence of letters: In printed material describing their meaning and use, prosigns are shown either as a sequence of dots and dashes for the sound of a telegraph, or by an overlined sequence of letters from the International Morse Code, which when sent without the usual spacing, sounds like the prosign symbol.

The best-known example of the convention is the standard distress call preamble: SOS. As a prosign it is not really composed of the three separate letters S, O, and S, (in International Morse:   ▄ ▄ ▄    ▄▄▄ ▄▄▄ ▄▄▄    ▄ ▄ ▄ ) but is run together as a single symbol   ▄ ▄ ▄ ▄▄▄ ▄▄▄ ▄▄▄ ▄ ▄ ▄ , which is a sign in its own right.

In the early decades of telegraphy, many efficiency improvements were incorporated into operations. Each of the early versions of Morse code was an example of that: With only one glaring exception (Intl. Morse O), they all encoded more common characters into shorter keying sequences, and the rare ones into longer, thus effecting online data compression. The introduction of Morse symbols called procedural signs or prosigns was then just a logical progression. They were not defined by the developers of Morse code, but were gradually introduced by telegraph operators to improve the speed and accuracy of high-volume message handling, especially those sent over that era's problematic long distance communication channels, such as transoceanic cables and later longwave wireless telegraphy.

Among other prosign uses, improvement in the legibility of written messages sent by telegraph (telegrams) using white space formatting was supported by the procedural symbols. To become an efficient telegraph operator it was important to master the Morse code prosigns, as well as the many standard abbreviations used to facilitate checking and re-sending sections of text.

There are at least three methods used to represent Morse prosign symbols:

1. Unique dot/dash sequences, e.g.   ▄▄▄ ▄ ▄ ▄ ▄▄▄  
2. Unique audible sounds, e.g. dah di di di dah 
3. Non-unique printed or written overlined character groups,^[a] e.g. BT

Although some of the prosigns as-written appear to be simply two adjacent letters, most prosigns are transmitted as digraphs that have no pauses between the patterns that represent the "combined" letters, and are most commonly written with a single bar over the merged letters (if more than one single character) to indicate this.^[4] The only difference between what is transmitted for the Morse code prosign vs. the separate letter signs is the presence or absence of an inter-letter space between the two "dit" / "dah" sequences. Although the difference in the transmission is subtle, the difference in meaning is gross:

For example, the unofficial prosign AA (  ▄ ▄▄▄ ▄ ▄▄▄ ) indicates that the receiving Morse operator should begin a new line,^[b] but the two separate letter sign or abbreviation AA (  ▄ ▄▄▄   ▄ ▄▄▄ ) indicates either the voice procedure words ALL AFTER, used to indicate what part of the previously transmitted message needs to be re-transmitted.^[c]

Because no letter boundaries are transmitted with the codes counted as prosigns, their representation by two letters is usually arbitrary, and may be done in multiple equivalent ways. Normally, one particular form is used by convention, but some prosigns have multiple forms in common use:

For example, AA (  ▄ ▄▄▄ +  ▄ ▄▄▄ ) is exactly equivalent to EK (  ▄ +  ▄▄▄ ▄ ▄▄▄ ) and RT (  ▄ ▄▄▄ ▄ +  ▄▄▄ ).

Although the well-known calling prosign for emergency messages SOS is always represented by the three letters "SOS", it could just as well be written VZE (  ▄ ▄ ▄ ▄▄▄ +  ▄▄▄ ▄▄▄ ▄ ▄ +  ▄ ), VGI (  ▄ ▄ ▄ ▄▄▄ +  ▄▄▄ ▄▄▄ ▄ +  ▄ ▄ ), or even 3B (  ▄ ▄ ▄ ▄▄▄ ▄▄▄ +  ▄▄▄ ▄ ▄ ▄ ); all of these render the same single code   ▄ ▄ ▄ ▄▄▄ ▄▄▄ ▄▄▄ ▄ ▄ ▄ .

Many Morse code prosigns do not have written or printed textual character representations in the original source information, even if they do represent characters in other contexts. For example, when embedded in text the Morse code sequence   ▄▄▄ ▄ ▄ ▄ ▄▄▄  represents the "double hyphen" character (normally "=", but also "– –").^[1] When the same code appears alone it indicates the action of spacing down two lines on a page in order to create the white space indicating the start of a new paragraph^[2] or new section in a message heading.^[1] When used as a prosign, there is no actual written or printed character representation or symbol for a new paragraph (i.e. no symbol corresponding to "¶"), other than the two-line white space itself.

Some prosigns are in unofficial use for special characters in languages other than English, for example AA is used unofficially for both the "next line" prosign^[b] and for "Ä",^[6][7] neither of which is in the international standard.^[1] Other prosigns are officially designated for both characters and prosigns, such as AR equiv. "+", which marks the end of a message.^[d][1] Some genuinely have only one use, such as CT or the equivalent KA (  ▄▄▄ ▄ ▄▄▄ ▄ ▄▄▄ ), the International Morse prosign that marks the start of a new transmission^[1] or new message.^[2]

The procedure signs below are compiled from the official specification for Morse Code, ITU-R M.1677, International Morse Code,^[1] while others are defined the International Radio Regulations for Mobile Maritime Service, including ITU-R M.1170,^[8] ITU-R M.1172,^[4] and the Maritime International Code of Signals,^[5] with a few details of their use appearing in ACP 131,^[9] which otherwise defines operating signals, not procedure signals.

The following table of prosigns includes K and R, which could be considered either abbreviations (for "okay, go ahead", and for "received") or prosigns that are also letters. All of the rest of the symbols are not letters, but in some cases are also used as punctuation.

The following table lists standard abbreviations used for organizing radiotelegraph traffic, however none of them are actual prosigns, despite their similar purpose. All are strictly used as normal strings of one to several letters, never as digraph symbols, and have standard meanings used for the management of sending and receiving messages. Dots following indicate that in use, the abbreviation is always followed by more information.

For the special purpose of exchanging ARRL Radiograms during National Traffic System nets, the following prosigns and signals can be used, most of which are an exact match with ITU-R and Combined Communications Electronics Board (military) standards; a few have no equivalent in any other definition of Morse code procedure signals or abbreviations.