base 11 is a positional numeral system with eleven as its base. It uses 11 different digits for representing numbers. The digits for base 11 could be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and a.

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.