In nearly every thin section of a rock containing monoelinic pyroxenes it will be found that one or more crystals twinned on the orthopinacoid (100) are included. In such crystals the twin-plane is marked very clearly by the difference of relative retardation and extinction, and also by the interference-bands if the twin-plane is inclined to the plane of section. For the purposes of the complete optical determination, it is essential for the proposed method that one half of the twin should exhibit the emergence of an optic axis suitable for the determination of the optic axial angle by the Becke method.