Alphanumeric characters were variable length and could use any character code of 8-bits or less.
This prompted the game to reveal a 24 character code.
Each three character code corresponded to an exchange within the London telephone area.
For example, when you type on your keyboard, it responds by generating different character codes.
Each I/O device type had its own more or less similar character code.
Some will produce the same character code as if the control key was not held down.
Keyboards also typically have a few single keys which produce control character codes.
Each video compression format is identified by a four character code.
The first three character codes can be set to zero.
The three character codes were dialled after an initial '0' which gave trunk access.