The encephalic circulation can be thought of as a double inlet vascular system with two parallel arterial pairs, the internal carotid arteries (ICA) and the vertebral arteries (VA), which together form the vertebrobasilar system, the former anterior and the latter posterior; and a single outlet for the foremost venous drain, the pair of internal jugular veins (IJVs). This complex layout is engineered so that each group of arteries, the ICA and the VA, respectively, is responsible for supplying the supratentorial space and the posterior fossa. In the latter, a “Rule of 3” of the posterior fossa has been described to correlate three portions of the brainstem, three fissures, three cerebellar peduncles, three cerebellar cortical surfaces, and three cranial nerve groups, all supplied by three respective arteries, forming three complexes (upper, middle, and lower) (Basma and Sorenson 2018). In other words, this so-called double inlet vascular system comprises simply the anterior and the posterior circulations, both of which are crucial components of the circle of Willis, each circuit with its territory, topography, singularities, and anatomical nuances. The circle of Willis lies in the basal aspect of the telencephalon and is responsible for supratentorial arterial blood supply.