A Schläfli Symbol is a notation to define regular polytopes and tilings. They appear in the form . The first X-1 entries define a polytope, and the Xth entry defines how many are around each X-2-dimensional subfacet. For example, defines a polytope where there are four triangles (schläfli symbol ) around each vertex; in this case, an octahedron. As another example, defines a polytope where there are four tetrahedra (schläfli symbol ) around each edge; in this case, a tetrarss. Regular polygons of X sides have the schläfli symbol .