Why a Phone Number Pad is Laid Out Like It Is
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The engineering factor was most likely the dual tone multi-frequency (DTMF) technology used for creating “touch tones”. To create 10 tones you need at least 7 frequencies (6 frequencies maxes out at 3 low and 3 high for only 9 combinations). With 7 frequencies, you can get 10 tones (5×2) but the max is 12 tones (3×4). The chosen design allowed engineers to add two buttons (* and #) to allow the maximum flexibility.