A cell phone keypad refers to the number and letter buttons found on a cell phone, for the purposes of dialing and typing. This type of push-button keypad was first used in the 1960s, when the dual-tone multi-frequency (DTMF) system was developed in order to replace a rotary dial. This means that pressing a button on the keypad causes two simultaneous tone frequencies; these frequencies are unique to each individual number, and are used to complete the call.
A traditional cell phone keypad, or keypad on any phone, is generally designed in a 3 x 4 pattern. This means that there are four horizontal rows, each including three buttons. The fourth row includes the asterisk and pound sign buttons.
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In addition, each button in this 3 x 4 grid pattern also includes corresponding letters. This is a fairly universal pattern, and generally features three letters per number, beginning with the number "2," which includes A, B, and C. The number "3" includes D, E, and F, and so on. These letters may be used to make phone numbers easier to remember; i.e., one might be able to place their business name right in their phone number. On a cell phone keypad, however, these numbers are used for texting and composing emails.
Because cell phones are becoming more and more complex, cell phone keypads are as well. A cell phone keypad might now feature an entire keyboard on the phone, in a compressed version. Each individual button will correspond to an individual letter. Some cell phones may feature one keypad for typing that slides out, and a separate one for dialing; many simply add numbers to the full-size keyboard, however. This can make it slightly more difficult to dial the "custom" numbers mentioned above, because the commonly used letters and numbers on the 3 x 4 grid will no longer correspond.
A cell phone keypad might also simply be a touchscreen keypad, able to switch back and forth between a full keyboard layout or a more traditional cell phone design. Other cell phone keypads might feature large, easy-to-read number buttons for those who are visually impaired or have difficulty dialing. Finally, cell phones designed for children might not feature numbers at all on the keypad, and will instead just include large buttons, such as a picture of a house for the child to call home, or a first aid sign for emergency calls.