A full understanding of the morphology of atrioventricular septal defects ('endocardial cushion defects,' 'atrioventricular canal malformations') demands knowledge of the exact nature of three specific breaches in the skirt of valve leaflet tissue guarding the atrioventricular junction. One of these is the space between the left ventricular components of the two leaflets that are enclosed in both the right and left ventricles (the bridging leaflets). Traditionally described as a cleft, it has been suggested more recently that this gap functions as a commissure. The second space is that which is found anteriorly and superiorly in the so-called Rastelli type A malformation. This is also called a cleft in a common anterior leaflet, but it has been suggested that it too is a commissure. The final breach is that produced by the surgeon when he divides the free-floating superior bridging leaflet if repairing the so-called Rastelli type C malformation with a one-patch technique. It is generally agreed that this breach be considered a division. To adjudicate the nature of the other breaches, it is necessary to compare them with commissures in atrioventricular and arterial valves and with the isolated cleft that exists in the aortic leaflet of a normal mitral valve. These considerations show that the gap between the left ventricular components of the bridging leaflets functions as a commissure even though it is not supported by a papillary muscle. This would not be expected, since the commissural attachments of the leaflets are in the left and right ventricle, respectively. The gap seen anteriorly in the Rastelli type A malformation is also a commissure, being supported in typical fashion by the medial papillary muscle of the right ventricle.