T9 is a text entry system using the 9 number keys - T9 stands for Text 9. It is most often used with cell phones.
Text entry using a numeric keyboard is well known to cell phone users. Typically 3 letters are assigned to each number and you tap once for the first letter, twice for the second letter and three times for the third. Some eBook Readers also use this method.
The idea behind T9 is that you will only need to tap a key once to get letters for a word. The secret is that it uses a built-in word list to figure out which of the 3 (or 4) letters is correct. As you type it will redo earlier letters if needed to make a word. If you need a number then tap and hold on the key until the number appears on the screen.
Of course, 9 keys are not quite enough in practice. You need a NEXT button to select from available choices if it picks the wrong word, a BACK button to fix an incorrect entry, and a SPACE button to tell it the word is done.
Only 8 keys are used for letters to match the standard telephone usage. The number 1 is used to enter punctuation.
A LEARN button allows you to add new words to the word list using traditional multi-tap key entry or just to make a word it doesn't know.
Another interesting way the T9 can be used is to look up a word in a dictionary from the book you are reading. This does not need a word list since it uses the page itself. When you see a word on the page you want to look up you just type the numbers as you would for any word and the program maps the numbers to a word on the page and then highlights the matching word. Tapping ok then looks up that word in the dictionary. A next button could be used to pick up a different matching word further down the page.
While T9 is prevalent since it makes use of the full number set there could be other systems as well. For example, a T5 system would work while requiring only 5 keys (plus some special keys such as next and ok). It would use 5 letters per key (6 for the last key) and would otherwise work the same as the T9 system. Of course it is more likely to mis-guess the word.
This mapping assumes a standard telephone style keypad and the existence of a few more special keys.
- 1 punctuation
- 2 ABC
- 3 DEF
- 4 GHI
- 5 JKL
- 6 MNO
- 7 PQRS
- 8 TUV
- 9 WXYZ
- * caps/caps lock
- # space
 Devices using T9
T9 is used most often in smartphones but can also be valuable for searching in eBook Readers. The following devices have T9 although may not have a full implementation:
Cool Reader uses the T5 variation for dictionary lookup.