You may run into the term “overscore” while studying the Morse code. This term is used to indicate a change in how characters in the Morse code are sent.
Prosigns are often used in sending Morse code as a means of conveying information in an abbreviated format. For example, you might send the letter ‘K’ to mean “I’m done sending, now it’s your turn to transmit. The letter ‘K’, in this case, is not used as a letter, but, is a prosign meaning “invitation to transmit”.
There are other prosigns in common use. Many prosigns are represented as digraphs, i. e., a combination of two letters. An ‘overscore’ is used to indicate that the two letters are actually a digraph and should be treated as one character. Overscore refers to a line drawn over the two letters.
A commonly used prosign is AS. If you were to send the letters ‘AS’, you would send: di-dah, pause 1 character time, di-di-dit. When you make it the prosign ‘AS’, which means “stand by; wait”, you would send it as: di-dah-di-di-dit. Notice that the character time pause is missing. The overscore tells you that both characters are to be sent as if they were one character with the codes of the two individual characters combined into one.
Other common prosign digraphs are AR, meaning “end of transmission” and sent as “di-dah-di-dah-dit”, SK, meaning “end of work” and sent as “di-di-di-dah-di-dah”, and HH, meaning “error” and sent as “di-di-di-di-di-di-di-dit”.
In CW traffic handling, you use the prosign AA as a separator between lines of the address or signature parts of a radiogram, and BT as a separator between the address part and text part and between the text part and signature part of a radiogram.
Amateur radiograms are usually sent as:
Preamble part Address Part line 1 AA Address Part line 2 AA Address Part line 3 AA BT Text Part BT Signature Part
The prosigns AA and BT are important in the transmission of a radiogram and aid in the accurate transmission of the radiogram.