Mechanical adding machines, based on rotating wheels, always have the 0 button adjacent to the 1 button. By convention, most old adding machines had the numbers increasing in value from the bottom. When the numbers were put onto a pad arranged as a 3 by 3 grid with one left over, the order of the numbers was kept the same.

On a rotary telephone dial, the 0 comes adjacent to the 9 because a 0 in the telephone number is signalled by 10 pulses on the line. When telephones acquired push buttons in a grid, the ordering of the buttons was carried over from the old telephone dial.

View Another Question?