Telephone

Before phones had keyboards, a text message was typed on the number keys. Each key carried three or four letters, and you pressed it until the letter you wanted came up: 2 for a, 22 for b, 222 for c.

Two letters on the same key in a row are a problem, because 2222 could be cab or it could be ab. Phones solved it by waiting a moment, or by offering a “next” key. Here that is a *, so cab is 222*2*22.

A # capitalises the next letter. Two of them switch to number mode, where a key press is just its own digit, and three turn on caps lock. Another # switches back.

This is not T9, which presses each key once and guesses the word.

Examples:

The layout is the one on an ordinary keypad: 1 holds the punctuation, 0 is the space bar, and abc through wxyz fill 2 through 9.