The pulmonary valve lies anterior and to the left of the aortic valve. The three pulmonary leaflets assume the shape of half moons (semi-lunar) and are similar but usually not equal in size. The right and left coronary sinuses of the aorta always face the pulmonary valve. The leaflets are thinner and more delicate than the aortic leaflets. Unlike the aortic valve, the pulmonary valve sits on a complete muscular ring of the infundibulum and is not in direct continuity with the tricuspid valve. It is thickest along the closing edge. The delicate pocket-like leaflets are formed primarily of collagen, and they, therefore, open and close passively, with little elastic recoil. In the middle of the free edge of each leaflet is a fibrous mound, the nodule of Arrantius. Coaption of the three nodules ensures complete central closure of the valve orifice during ventricular diastole.