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 21 is a positional numeral system with twenty-one as its base. It uses 21 different digits for representing numbers. The digits for base 21 could be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, f, g, h, i, j, and k.