The term Horsley-Clarke zero plane denotes a horizontal plane for brain stereotaxis based on cranial landmarks. It is parallel to, and 10mm superior to the orbitomeatal plane, which passes through the inferior margins of the orbits and the interaural line ( Clarke-1920 ). It was the most common horizontal zero plane for brain atlases of nonhuman primates until noninvasive methods, such as ventriculography and magnetic resonance imaging, made it possible to visualize internal landmarks in the living brain. The zero horizontal plane for stereotaxis based on those methods is perpendicular to the midsagittal plane and passes through the bicommissural line ( Martin-2000 ).