If we're being more technical, clocks are an example of modulo arithmetic. In modulo 12 there are only 12 integers, and when you count higher it circles around to the beginning. In theory mod 12 arithmetic would work equally well if you counted from 0 to 11 or from 1 to 12, but the convention is to go from 0 to 11 because it makes math a lot easier. For example, 0*0=0, 0*1=0, 0*2=0, etc. which follows the same rules we're used to. But if you said that 12*12=12, 12*1=12, 12*2=12, etc. that would be weirder and less intuitive. Another way to think of mod 12 is that a number is the remainder when it's divided by 12, and if a number divides evenly into 12 its remainder is zero. (e.g. 72/12 is 6, not 5 with remainder 12) It also makes decimals more intuitive, because numbers like 0.5, 0.273, 0.689523 fall between 0 and 1, so it makes sense for them to begin with 0. If they fell between 12 and 1, it would be confusing to have 12 followed by decimals beginning with 0 (but no actual 0), or alternatively, even more confusing to have 12.5, 12.273, 12.689523 followed by 1 (which is the way the clock works, and it makes no sense at all.) This is easier to understand in mod 10, where a number is equivalent to its last digit (and decimal places). 21 becomes 1, 20 becomes 0, 20.689523 becomes 0.689523. When viewed mathematically this convention makes much more sense.