In mathematical numeral systems, the base or radix for the simplest case is the number of unique digits, including zero, that a positional numeral system uses to represent numbers. For example, for the decimal system (the most common system in use today) the radix is ten, because it uses the ten digits from 0 through 9. In any numeral system, the base is written as "10". In a base ten numeral system, "10" represents the number ten; in a base two system, "10" represents the number two.