The technique for measuring the interfacial angles of crystals. The instrument used, a goniometer (or contact goniometer) resembles a 180° protractor with a pivoted straight edge at the point of origin. The angle between the normals to crystal faces is then read directly from the graduated scale. More accurate measurements on very small crystals may be made using an optical system and a telescope (optical goniometry), with the crystal mounted on a rotating stage fitted with a vernier scale. These instruments are known as ‘one-circle’ or ‘two-circle’ reflecting goniometers. The crystal faces are arranged with the crystal edges parallel to the axis of rotation of the goniometer head.