Digits are symbols in combinations to represent numerals, and numerals represent ideas of numbers. This is similar to how letters are symbols in combination to represent words, and words represent ideas of things. Aside from its primary purpose, digits are also studied in the field of number theory.
In any number system, the most common of which is the decimal system, the number of digits required is always equal to the absolute value of the base. The decimal system has the 10 digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9, whereas the binary system has the two digits 0 and 1.
Number theory investigates various aspects of digits. For instance, consider the digital root. This is the process of summing the digits of a given number, then summing the digits of the result, and so forth until a single-digit number is reached. The digit obtained is called the digital root, and the number of additions required to obtain it is called the additive persistence.