With regards to the overall proportionality of the face, the interrelationship of the widths of the components of the face is an important consideration. The ‘rule of fifths’ describes the ideal transverse proportions of the face to comprise equal fifths, each segment roughly equal to one eye width.
The central fifth of the face is outlined by the inner canthi of the eyes. The inner canthus of the eye is the inner corner of the eye containing the lacrimal duct. The inner canthus must be coincident with the ala of the base of the nose if you draw a vertical line across.
Regarding the medial two fifths of the face, a vertical line from the outer canthi of the eyes should be coincident with the gonial angles of the mandible. Lastly, the outer two fifths of the face are measured from the base of ear to the helix of the ear, which represents the width of the ears.
