RTTY is a very simple technique which uses a five-bit code to represent all the letters of
the alphabet, the numbers, some punctuation and some control characters.
The 5 data bits allow for only 32 different codes, which cannot accommodate the 26 letters, 10 figures, space, a few punctuation marks and the required control codes, such as carriage return, new line, bell, etc.
To overcome this limitation, the teleprinter has two states, the unshifted or letters state and the shifted or numbers or figures state.
The change from one state to the other takes place when the special control codes LETTERS and FIGURES are sent from the keyboard or received from the line.
In the letters state the teleprinter prints the letters and space while in the shifted state it prints the numerals and punctuation marks.
There is no form of error correction provided in RTTY thus noise and interference can make communication
Lots of information for RTTY can be found on RTTY.com