A Coxeter group, represented by Coxeter diagram , is given Coxeter notation [p,q] for the branch orders. Each node in the Coxeter diagram represents a mirror, by convention called ρ i (and matrix R i). The generators of this group [p,q] are reflections: ρ 0, ρ 1, and ρ 2. Rotational subsymmetry is given as products of reflections: By convention, σ 0,1 (and matrix S 0,1) = ρ 0 ρ 1 represents a rotation of angle π/p, and σ 1,2 = ρ 1 ρ 2 is a rotation of angle π/q, and σ 0,2 = ρ 0 ρ 2 represents a rotation of angle π/2.