Our decimal number system is known as a positional number system, because the value of the number depends on the position of the digits. For example, the number 123 has a very different value than the number 321, although the same digits are used in both numbers.
(Although we are accustomed to our decimal number system, which is positional, other ancient number systems, such as the Egyptian number system were not positional, but rather used many additional symbols to represent larger values.)
In a positional number system, the value of each digit is determined by which place it appears in the full number. The lowest place value is the rightmost position, and each successive position to the left has a higher place value.
In our decimal number system, the rightmost position represents the "ones" column, the next position represents the "tens" column, the next position represents "hundreds", etc. Therefore, the number 123 represents 1 hundred and 2 tens and 3 ones, whereas the number 321 represents 3 hundreds and 2 tens and 1 one.
The values of each position correspond to powers of the base of the number system. So for our decimal number system, which uses base 10, the place values correspond to powers of 10 (The symbol ^ is used for exponentiation, eg. 5^2 means 5 squared; 5^3 means 5 cubed):
| ... | 1000 | 100 | 10 | 1 |
| ... | 10^3 | 10^2 | 10^1 | 10^0 |
Other number systems use different bases. The binary number system uses base 2, so the place values of the digits of a binary number correspond to powers of 2. For example, the value of the binary number 10011 is determined by computing the place value of each of the digits of the number:
| 1 | 0 | 0 | 1 | 1 | the binary number |
| 2^4 | 2^3 | 2^2 | 2^1 | 2^0 | place values |
| (1 * 2^4) | + | (0 * 2^3) | + | (0 * 2^2) | + | (1 * 2^1) | + | (1 * 2^0) | |
| = | 16 | + | 0 | + | 0 | + | 2 | + | 1 |
| = | 19 | ||||||||