base 36 is a positional numeral system with thirty-six as its base. It uses 36 different digits for representing numbers. The digits for base 36 could be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, and z.

conversion table

base 36	base 21	base 36	base 21
1	≈ 1	b	≈ b
2	≈ 2	c	≈ c
3	≈ 3	d	≈ d
4	≈ 4	e	≈ e
5	≈ 5	f	≈ f
6	≈ 6	g	≈ g
7	≈ 7	h	≈ h
8	≈ 8	i	≈ i
9	≈ 9	j	≈ j
a	≈ a	k	≈ k