E.161 is an ITU-T Recommendation that defines the arrangement of digits, letters, and symbols on telephone keypads and rotary dials. It also defines the recommended mapping between the basic Latin alphabet and digits (e.g., "DEF" on 3). Uses for this mapping include:
- Multi-tap and predictive text systems.
- Forming phonewords from telephone numbers.
- Using alphabetic characters (e.g. as a mnemonic) in a personal identification number.
Keypads are specified both in the common 4 × 3 and several variations, such as 6 × 2 and 2 × 5. E.161 also specifies the dimensions and characteristics of the star and square, referred to in the standard as the 'star' and 'square' keys, respectively. (In practice, the 'square' key is almost invariably replaced by the number sign. and the sextile symbol is the more common choice for the star key.)
