A petrie subgroup of an irreducible coxeter group can be created by the product of all of the generators. It can be seen in the skew regular petrie polygon of a regular polytope. The order of the new group is called the Coxeter number of the original Coxeter group. The Coxeter number of a Coxeter group is 2 m / n, where n is the rank, and m is the number of reflections. A petrie subgroup can be written with a π superscript. For example, [3,3] is the petrie subgroup of a tetrahedral group, cyclic group order 4, generated by a rotoreflection. A rank 4 Coxeter group will have a double rotation generator, like [4,3,3] is order 8.