The number of digits in a number system is equal to the base of
the system. The decimal system is base 10 and has ten digits.
Binary has two bits, which is short for binary digits. Hexadecimal
has sixteen digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E
& F), and so on.