The design ofcell phone keypads corresponds to the normal dialing keypads specified by the International Telecommunication Union(ITU) and is primarily used to enter the telephone number of the communication partner. In addition, cell phones have various keys for menu controland navigation, selection keys for services, and keys for establishing and terminating calls.
The numeric keys of cell phone keypads are arranged in four rows and three columns. The top row contains the digits 1, 2 and 3, the next row contains the digits 4, 5 and 6, the third row contains the digits 7, 8 and 9, and the fourth row contains the special character *, the digit 9 and the special character hash (#). The number keys are used to enter the phone number of the desired subscriber and then to start the connection setup.
Most of the number keys, with the exception of the number keys "0" and "1", are additionally assigned letters. The letter assignment of the numeric keys is also defined by the ITU as Standard Ambiguous Code( SAC). The letter assignment can also be used to enter short texts for the short message service( SMS) on a cell phone keypad. Most of the numeric keys have three letters stored, the numeric keys "7" and "9" have four. The numeric keys "0" and "1" do not have a letter background but a function assignment. The numeric key "2" is assigned the letters "ABC", the "3" the letters "DEF", the "4" the letters "GHI", etc. up to the numeric key "9" which is assigned the letters "WXYZ".
When a letter is entered, this is done by pressing the corresponding numeric key several times. How often this must be pressed depends on the position of the letter. For example, if the letter "F" is to be entered and the numeric key "3" is assigned to it, then the "3" key must be pressed three times because "F" is in the third position. This makes entering a word relatively time-consuming. For example, if the word itwissen is to be entered, then the keystroke looks like this: 4448944477773366.
Since the input is relatively laborious, there are programs that support a faster input, where each letter is entered with a digit. T9 is such a text input assistant, which works with statistically probability stored word inputs.