The ten digits of the decimal number system, in the order they appear at the top row of a standard keyboard. The reason they are placed this way, rather than in order of ascending magnitude is generally unknown. One theory is that the '0' was originally a '10' key; by the time it discovered that zero appears in numbers such as 20 and 500, people had gotten so used to the positions of the first nine numbers that they refused to let them be moved.