T9, which stands for Text on 9 keys, is a patented predictive text technology for mobile phones, originally developed by Tegic Communications, now part of Nuance Communications.
T9 is used on phones from LG, NEC, Nokia, Samsung Electronics, Siemens, Sony Ericsson, Sanyo, Sagem and others. It was also used by Texas Instruments PDA Avigo during the late '90s. Its main competitors are iTap created by Motorola, SureType created by RIM, Eatoni's LetterWise and WordWise, and Intelab's Tauto.
T9's objective is to make it easier to type text messages. It allows words to be entered by a single keypress for each letter, as opposed to the multi-tap approach used in the older generation of mobile phones in which several letters are associated with each key, and selecting one letter often requires multiple keypresses.
It combines the groups of letters on each phone key with a fast-access dictionary of words. It looks up in the dictionary all words corresponding to the sequence of keypresses and orders them by frequency of use.
As it gains familiarity with the words and phrases the user commonly uses, it speeds up the process by offering the most frequently used words first and then lets the user access other choices with one or more presses of a predefined Next key.
The dictionary can be expanded by adding missing words, enabling them to be recognized in the future. After introducing a new word, the next time the user tries to produce that word T9 will add it to the predictive dictionary.
The user can manually add words to the UDB (user database) via multi-tap. The implementation of the user database depends on the version of T9 and how T9 is actually integrated on the device. Some phone manufacturers implement a permanent user database, while others implement one for the duration of the session.