Understanding the anatomy of septal defects is greatly facilitated if the heart is thought of as having three distinct septal structures: the atrial septum, the atrioventricular septum, and the ventricular septum (Figure 8.1.1). The normal atrial septum is relatively small. It is made up, for the most part, by the floor of the oval fossa. When viewed from the right atrial aspect, the fossa has a floor, surrounded by rims. As we have shown in Chapter 2, the floor is derived from the primary atrial septum, or septum primum. Although often considered to represent a secondary septum, or septum secundum, the larger parts of the rims, specifically the superior, antero-superior, and posterior components, are formed by infoldings of the adjacent right and left atrial walls.1 Infero-anteriorly, in contrast, the rim of the fossa is a true muscular septum (Figure 8.1.2).