Morse code prosigns or procedural signals are dot/dash sequence symbols that do not represent written characters, but instead are used to manage transmission and formatting of messages.  Prosigns are symbols that have specific functions, such as indicating changes of transmission communications protocol status and indicating (or initiating) textual white space formatting. Morse prosigns play a role similar to the role played by the nonprinting control characters of teleprinter and computer character set codes such as Baudot or ASCII. Morse code prosign symbols have been used since the 1860s.
In the early decades of telegraphy many operating efficiency improvements were incorporated into telegraph operations, including the introduction of Morse symbols known as procedure signs or prosigns. Prosigns were not defined by the inventors of Morse code, but were gradually introduced over time, and greatly improved the speed and performance of daily high-volume message handling operations.
Improvements to the legibility of formal written telegraph messages (telegrams) by means of white space formatting were thus supported by the creation of the additional new procedure symbols. With one exception, namely the Morse symbol for the alphabetic letter "K" (▄▄▄▄▄▄▄▄▄▄▄▄), which also serves as a prosign when used alone at the end of a transmission, the Morse code prosigns are unique dot/dash sequence code symbols. Mastery of these special Morse code prosigns is an important part of becoming a fluent and efficient telegrapher/telegraphist.
Prosigns may be represented in printed material either by a sequence of dots and dashes, or by a sequence of letters, which, if sent without the normal inter-character spacing (concatenated), correspond to the prosign symbol.
For example, the Morse code dot/dash sequence (▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄) which 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. Other than the two line white spaces themselves, there is no actual written or printed character representation or symbol for a new paragraph. Thus, in general, typical Morse code prosigns do not have written or printed textual character representations in the original source information.
Of methods used to represent Morse prosign symbols there are at least three:
|Prosign||Code Symbol||Meaning||Comments||Memory Aid||Verbalization|
|AA||▄▄▄▄▄▄▄▄▄▄▄▄▄▄||New Line (space down one line)||Typewritten as Carriage Return, Line Feed (CR-LF).||"Add A line"||"didahdidah"|
|AR||▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄||New Page (space down several lines)||Message separator. Single-line display may use printed "+".||"All Rendered"||"didahdidahdit"|
|AS||▄▄▄▄▄▄▄▄▄▄▄▄▄▄||Wait||Respond with: SN, or characters "R" (Roger) or "C" (Confirm).||"Wait A Sec"||"didahdididit"|
|BT||▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄||New Paragraph (space down two lines)||Typewritten CR-LF-LF. Single-line display may use printed "=".||"Begin Two"||"dahdidididah"|
|CT||▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄||Attention||Sometimes written as KA. Commencing important transmission.||"Copy This"||"dahdidahdidah"|
|HH||▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄||Error (Sometimes "????" is used.)||Sometimes written EEEEEEEE. Always followed by correct text.||"Error"||"didididididididit"|
|K||▄▄▄▄▄▄▄▄▄▄▄▄||Invitation for any station to transmit||Lone alphabetic character "K" at the end of a transmission.||"OK, go ahead"||"dahdidah"|
|KN||▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄||Invitation for named station to transmit||Go ahead, specific named station.||"OK, Named"||"dahdidahdahdit"|
|NJ||▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄||Shift to Wabun code||Shift from Morse code to Wabun code Kana characters.||"Next Japanese"||"dahdididahdahdah"|
|SK||▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄||End of contact||Sometimes written as VA.||"Silencing Key"||"didididahdidah"|
|SN||▄▄▄▄▄▄▄▄▄▄▄▄▄▄||Understood||Often written VE. Alternatively shift from Wabun to Morse code.||"Sho' 'Nuff"||"didididahdit"|
|SOS||▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄||International distress signal||Signals imminent danger to life or property. ( listen (help·info))||"Save Our Souls"||"didididahdahdahdididit"|
|BK||▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄||Break (Morse abbreviation)||Abbreviation for "back-to you".||"BreaK"||"dahdididit dahdidah"|
|CL||▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄||Closing (Morse abbreviation)||Abbreviation for "closing station".||"CLosing"||"dahdidahdit didahdidit"|
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