So then, find the principle axis and then look for mirror planes. If there is a mirror plane that contains the principle axis, it is a σv (or σd). If there is a mirror plane that is perpendicular to the principle axis, this is a σh. The distinction between the vertical and dihedral planes is that the latter bisects two rotation axes that are both perpendicular to the principle axis. The dihedral mirror planes are special types of vertical mirror planes. They are typically observed in conjunction with horizontal mirror planes, in molecules such as benzene.