There is a paucity of information in the literature regarding the quadrate ligament and the information that does exist is extremely conflicting. We dissected 30 cadavers (60 sides) to determine the morphology and function of this enigmatic ligament. A quadrate ligament (thickening of the elbow joint capsule) was found in all specimens. In all specimens this band was distinct from the circumferential fibres of the annular ligament. The length, width, and thickness of the quadrate ligament were found to be 11 mm, 8 mm, and 1 mm respectively. This ligament not only aided in securing the neck of the radius to the ulna but also resisted excessive supination and, to a lesser degree, pronation of the forearm. Following transection of the quadrate ligament, the head of the radius was secured to the ulna considerably less firmly and supination and pronation increased by 10 to 20 degrees and 5 to 8 degrees respectively. The quadrate ligament contributes to proximal radioulnar stability, limits the "spin" of this joint, and should be considered in manipulation, surgery, or imaging of the proximal forearm.