**Number Digits** (0, 1, 2, 3, 4, 5, 6, 7, 8, and 9) can be used to
represent any number; these symbols are the *decimal number system*.
When only one number is used (0, 1, 2, 3, 4, 5, 6, 7, 8, and 9) we have a *one-digit*
number.

The following are examples of *two-digit* numbers: 10, 11, 12, ... 99.

You see that two digit symbols are put together to represent a larger number than any one digit number. Three digit numbers such as 135 is greater than any two digit number such as 99. The idea continues forever.