A and B, the SCA arises at the midbrain level and encircles the brainstem near the pontomesencephalic junction. The SCA courses below the oculomotor and trochlear nerves and above the trigeminal nerve. The SCA loops down closer to the trigeminal nerve in B than in A. The AICA arises at the pontine level and courses by the abducens, facial, and vestibulocochlear nerves. In A, both AICAs pass below the abducens nerves. In B, the left abducens nerve passes in front of the AICA and the right abducens nerve passes behind the AICA. The PICAs arise from the vertebral artery at the medullary level and course in relation to the glossopharyngeal, vagus, accessory, and hypoglossal nerves. The origin of the SCAs are quite symmetrical from side to side. There is slight asymmetry in the level of origin of the AICAs and marked asymmetry in the level of the origin of the PICAs, especially in A. (Images courtesy of AL Rhoton, Jr.)