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The conventional way of writing numbers as a string of digits in which each digit, D, has value D * R^I, where R is the radix or (number) base and I is the digit's position counting leftward from zero at the least significant (right-hand) end. Each digit can be zero to R-1. Each position has a weight or significance R times greater than the position to its right and the right-most place has a weight of one.
Decimal numbers are radix ten, binary numbers are radix two, octal radix eight and hexadecimal radix 16.
Positional representation makes arithmetic operations on large numbers much easier than, say, roman numerals. It is fundamental to the binary representation used by digital computers.