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

base 15 is a positional numeral system with fifteen as its base. It uses 15 different digits for representing numbers. The digits for base 15 could be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, and e.