prosthetic reference plane that is determined both on the skull (skeleton) as well as on the head.
- Skull: Upper edge of the bony auditory canal (Porus acusticus externus) and lowest point of the lower edge of the bony eye socket (Orbita),
- Head: Center of the tragus (prominent cartilage of the auricle in front of the external auditory canal) and lowest point of the lower edge of the bony orbit covered by the facial skin
