The reflection operation is carried out with respect to symmetry elements known as planes of symmetry or mirror planes. Each such plane is denoted as  (sigma). Its orientation relative to the principal axis of the molecule is indicated by a subscript. The plane must pass through the molecule and cannot be completely outside it.
*If the plane of symmetry contains the principal axis of the molecule (i.e., the molecular -axis), it is designated as a vertical mirror plane, which is indicated by a subscript  ().
*If the plane of symmetry is perpendicular to the principal axis, it is designated as a horizontal mirror plane, which is indicated by a subscript  ().
*If the plane of symmetry bisects the angle between two 2-fold axes perpendicular to the principal axis, it is designated as a dihedral mirror plane, which is indicated by a subscript   ().
Through the reflection of each mirror plane, the molecule must be able to produce an identical image of itself.