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.

base 19 is a positional numeral system with nineteen as its base. It uses 19 different digits for representing numbers. The digits for base 19 could be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, f, g, h, and i.