Basion

In order to determine the distance of the zygomaxillare and the first molar from the basion (or prosthion), isosceles triangles were constructed from the measurements, which on one hand had the basion for the apex, the distance connecting both molar middle points as the base and the basion-first molar distance as the legs (Fig. 4. A), and on the other hand they had the distance between both zygomaxillaria as the base and the basion-zygomaxillaria as the base and the basion-zygomaxillary distance as the legs (Fig. 4. B).

Source: wiktionary

Additional skull measurements, taken with small sliding calipers, were from zygomaxillare, the lowest point on the zygomatic-maxillary suture, to the points of alveolare, basion and nasion.

Source: wiktionary

The tectorial membrane limits extension, while odontoid impaction on the basion limits flexion.

Source: wiktionary