The location, and morphology, of the superior sinus venosus interatrial communication remains contentious. As part of a clinical study, we examined anatomic specimens and echocardiograms so as to clarify the arrangement of the normal atrial septal structures, and compared them with the arrangement found in the superior sinus venosus defect. The pathognomonic diagnostic criterion in the abnormal hearts was overriding of the intact muscular rim of the oval fossa by the mouth of the superior caval vein. This muscular rim is, in reality, a tube of myocardium which encloses a core of extracardiac adipose tissue. Understanding of this anatomic conundrum clarifies the understanding of the structures of both the normal atrial septum and sinus venosus defects.