t9 is a predictive text entry system that is optimized for use with cellular phone keypads, developed by Tegic Communications.

When entering letters on a conventional phone keypad, you must somehow differentiate between which of the 3 or more letters that you want to use. For example, pressing the 5-key could mean you wanted a J, K, or L. Normally, you must either press the same key multiple times (twice to get a K, five times to get a K in the opposite case, seven times to actually just get the number 5), or you press an alternate key, like the * and # keys to select the left or right letter. So to type the word IF, you must usually type 444333 or 4#3#.

The t9 system appears to take the numbers, parse them out for all combinations, then match them against a dictionary. For example, to type the word "IF", you type:

 4  3
 G  D
 H  E
 I  F
You could mean nine different two letter combinations: GD, GE, GF, HD, HE, HF, ID, IE, and IF. By matching with a dictionary, you can see that only a couple of these make sense. 'He' and 'if' are the most likely, and it chooses one. If you want another spelling, you can then choose a different interpretation, much like the Microsoft IME that comes with Windows for entering Japanese and Chineese Kanji characters. You can also add words to its dictionary, to make words quicker to use next time you use them.

It is damn useful.