The lateral structures of the knee can be divided into three distinct layers. The deepest layer, the lateral part of the capsule, divides into two laminae just posterior to the overlying iliotibial tract. These laminae encompass three ligaments: the lateral collateral, the fabellofibular, and the arcuate ligaments. Three major anatomical variations are seen: reinforcement of the capsule by the arcuate ligament alone (13 per cent), by the fabellofibular ligament alone (20 per cent), and by both ligaments (67 per cent). One can predict which variation will be seen in a given patient from the information provided by a radiograph and by physical examination.