base 9 is a positional numeral system with nine as its base. It uses 9 different digits for representing numbers. The digits for base 9 could be 0, 1, 2, 3, 4, 5, 6, 7, and 8.

base 18 is a positional numeral system with eighteen as its base. It uses 18 different digits for representing numbers. The digits for base 18 could be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, f, g, and h.