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 there is an ambiguity even in context since they are not distinct.
Overview 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". 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.
Representation in printed material 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.
History 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.
Notation and representations There are at least three methods used to represent Morse prosign symbols:
Unique dot/dash sequences, e.g. ▄▄▄ ▄ ▄ ▄ ▄▄▄ Unique audible sounds, e.g. dah di di di dah Non-unique printed or written overlined character groups, 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. 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 significant:
For example, the unofficial prosign AA ( ▄ ▄▄▄ ▄ ▄▄▄ ) indicates that the receiving Morse operator should begin a new line, but the two separate letter sign or abbreviation AA ( ▄ ▄▄▄ ▄ ▄▄▄ ) indicates the voice procedure words ALL AFTER, used to indicate what part of the previously transmitted message needs to be re-transmitted. 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:
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