Surgical descriptions of the aortic root are not always in keeping with anatomy as observed in the autopsied heart. Although all surgeons appreciate that the concept of the aortic annulus does not imply the presence of a straight ring as a hinge point, the nature of the supporting fibrous structures relative to the semilunar attachment of the leaflets has yet to be clarified. We have analyzed 50 normal aortic roots, two fetal aortic roots sectioned histologically in horizontal and sagittal planes, respectively, and two autopsied adult hearts in which prosthetic aortic valves had been inserted during life. Our results demonstrate the important interrelationships between aortic sinuses, valvar leaflets, and supporting left ventricular structures that produce the three fibrous interleaflet triangles. It is the structure and location of these triangles that is the key to the understanding of the surgical anatomy. Our results also show that the presently used definition of commissure does not reflect adequately the extent of the zones of apposition between adjacent valvar leaflets, essential for normal function when the valve is closed.