Multi-tapping is an older procedure used to enter text using a telephone keypad. You tap (press) a key a certain number of times to select a letter from the ones associated with a particular key. For example, the letters A, B, and C are associated with the 2 key. Tapping the key once enters an "A," twice enters a "B," and three times enters a "C." Although multi-tapping is workable, it is also a bit cumbersome in practice, and a number of alternatives have been developed. The Fastap keypad is a miniature alphanumeric keypad for use with mobile phones. Another widely-implemented approach, text on nine keys (T9) uses predictive software that allows you to tap a key only once for each letter. T9 makes informed guesses about what text you intend to enter, based on the likelihood of letter combinations.
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