The basion is part of the human body's musculoskeletal system and is located on the skull. Anatomically it can be identified as the midpoint on the anterior margin of the foramen magnum on the occipital bone. It is a cephalometric landmark.
The opisthion is the median (midline) point of the posterior margin of the foramen magnum.
It is one of the skull landmarks, craniometric points for radiological or anthropological skull measurement.