Sites commonly marked on the scalp before applying the drapes include the coronal, sagittal, and lambdoid sutures; the central sulcus and sylvian fissures; and the pterion, inion, asterion, and keyhole. Approximating the site of the sylvian fissure and central sulcus on the scalp begins by not-ing the position of the nasion, inion, and frontozygomatic point. The nasion is located in the midline at the junction of the nasal and frontal bones. The inion is the site of a bony prominence that overlies the torcular herophili. The frontozygomatic point is located on the orbital rim 2.5 cm above the level where the upper edge of the zygomatic arch joins the orbital rim and just below the junction of the lateral and superior margins of the orbital rim. The next steps are to construct a line along the sagittal suture and, using a flexible measuring tape, to determine the distance along this line from the nasion to inion and mark the midpoint and three-quarter points (50% and 75% points). The sylvian fissure is located along a line that extends backward from the frontozygomatic point across the lateral surface of the head to the three-quarter point. The pterion, the site on the temple approximating the lateral end of the sphenoid ridge, is located 3 cm behind the frontozygomatic point on the sylvian fissure line. The central sulcus is located by identifying the upper and lower rolandic points. The upper rolandic point is located 2 cm behind the midpoint (50% plus 2 cm point) on the nasion-to-inion midsagittal line. The lower rolandic point is located where a line extending from the midpoint of the upper margin of the zygomatic arch to the upper rolandic point crosses the line defining the sylvian fissure. A line connecting the upper and lower rolandic points approximates the central sulcus. The lower rolandic point is located approximately 2.5 cm behind the pterion on the sylvian fissure line. Another important point is the keyhole, the site of a burr hole, which if properly placed, has the frontal dura in the depths of its upper half and the periorbita in its lower half. It is approximately 3 cm anterior to the pterion, just above the lateral end of the superior orbital rim and under the most anterior point of attachment of the temporalis muscle and fascia to the temporal line. (Images courtesy of AL Rhoton, Jr.)