CoachA mirror plane is a plane through which reflection is a symmetry operation; . A mirror plane is a vertical mirror plane if it contains the principal axis, a horizontal mirror plane if it is perpendicular to it, and a dihedral mirror plane if it contains the principal axis and bisects the angle between two axes perpendicular to it.
Examples
Example 4.19 (An achiral molecule without a plane)
Some molecules have neither a mirror plane nor a centre of inversion, yet are achiral because of an axis: the classic case is a spiro compound built from two identically substituted rings at right angles, of point group . Chirality cannot be judged by looking for a plane alone.