The external occipital fissure (EOF) corresponds to the extension of the Parieto-Occipital Sulcus (POS) into the brain convexity. It is usually a deep transversal sulcus on the medial side of each hemisphere.

The EOF/POS meeting point is a useful surgical landmark because it defines the parieto-occipital sulcus wich divide de precuneus (parietal) fron the cuneus (occipital) .

This sulcal key point (EOS/POS) lies underneath each paramedian area corresponding to the angle between the sagittal and lambdoid suture. The craniometrical position of Lambda in adults is around 25 cm posterior to the Nasion, 13 cm posterior to the Bregma and 3 cm superior to the Opisthocranion (not to be confused with the Inion).