That is, in Figure 9a, an octahedron face would be an equilateral triangle at the intersection of the three dodecahedron faces where the twin axis emerges.
To meet the above conditions, it is sufficient to show that faces of the form lie in the zone [111], i.e., are parallel to the twin axis. To show that a face (hkl) lies in the zone [uvw], the zonal equation
Symmetry considerations then show that, if one face of the form satisfies the above equation there will exist a belly band of six (or twelve) faces which lie in the zone of the twin axis. Forms of the hexoctahedral class of the isometric system have the symmetry 4/m 3 2/m,.