Morse code is a system that transmits text as unique sounds or lights. It was developed in the 1800s by Samuel Morse to be used with the telegraph. Each letter A–Z and digit 0–9 has its own unique dot-dash code, where dots are short signals (or pulses of light) and dashes are long signals. In the case of the letter A, the symbol is made up of a single dot followed by a single dash: · −.
This pattern is called "di-dah" and consists of one dot (a short signal) followed by one dash (a longer signal). The timing of the letter A in Morse code follows the same pattern as any other letter; the dot comes first, followed by the dash.
Why the pattern is short
Alfred Vail, who later worked with Morse on the telegraph, designed the codes so that more commonly used letters would receive shorter sequences. Because there are fewer symbols than possible combinations of dots and dashes, you get more dots than dashes, allowing for a quicker transmission. Letters such as E, I, S, etc., are among the most used, while Y, J, Q, and others are among the least. This is why E is represented by just one short dot (·), and the less-used letters get longer sequences.
How A appears in radio prosigns
In real radio prosigns, the A pattern can be extended into something like AR, which is used to signify the end of a message. As with the letter A itself, it's a series of three symbols: dot, dash, and then dot again. Radio operators will use prosigns when they're transmitting over the airwaves, and if someone needs to communicate over a distance — say, between ships at sea — these prosigns help make sure that messages are transmitted properly.
The letter A also appears at the beginning of other prosigns. AS is sent as the letters A and S run together without the normal letter gap (.-...) and means wait. AA is likewise sent without an internal letter gap (.-.-) and is used for a new line. Because prosigns are continuous patterns, they should not be explained as merely changing the length of one dot.