The Frankfurt plane (also called the auriculo-orbital plane) was established at the World Congress on Anthropology in Frankfurt, Germany in 1884, and decreed as the anatomical position of the human skull. It was decided that a plane passing through the inferior margin of the left orbit (the point called the left orbitale) and the upper margin of each ear canal (the point called the porion) was most nearly parallel to the surface of the earth, and also close to the position the head is normally carried in the living subject.
Note that in the normal subject, both orbitales and both porions lie in a single plane. However, due to pathology, this is not always the case. The formal definition specifies only three points, which must all lie on a plane.
For the purposes of comparison, the skulls of other species, notably hominids and primates may be studied in the Frankfurt plane, but it is not considered to be the anatomical position for other species.
The Frankfurt plane may also be used as a reference point in related fields. For example, in prosthodontics, the Frankfurt-Mandibular plane Angle (FMA) is the angle formed at the intersection of the Frankfurt plane with the mandibular plane.