The common element in all numeric pads is ‘0’ button at the bottom.The remaining numbers go from top to the bottom in a phone whereas from bottom to the top in calculators. There is a reason for this.

The most underused buttons lies on the bottom. Mechanical cash registers in the old days were designed accordingly as well. 0 being at the bottom because it was of low usage. The designers thought that it must be accessible to the fingers easily and visible to eyes just at a glance. And then the numbers followed i.e. 1-2-3 in the next row, followed by 4-5-6 in the row above it, and finally 7-8-9. For electronic calculators the design was in a way that used the existing calculator’s keypad arrangement with ‘0’ at the bottom, ‘1’ at the bottom left corner, and ‘9’ in the top right corner.

Do you remember the old phone ? the one that you had to put a finger inside and spin to dial. The touch phone design totally changed in an opposite way to the rotary dial rather than replicating the design of the calculators.

After several testings they came to a conclusion that the lowest error rate was that a three-by-three matrix with 1, 2, 3 at the top.