Fischgold’s bimastoid line

FDGL=Fischgold’s digastric line, FBML= Fischgold’s bimastoid line, x+y=total overhang of C1 on C2.

see Rule of Spence.

joins tips of mastoid processes. The odontoid tip averages 2 mm above this line (range: 3 mm below to 10 mm above) and this line should cross the atlanto-occipital joint.