In this fourth part of our series of articles concerned with the surgical anatomy of ventricular septal defects (VSDs), we have analyzed the arrangements when both arterial trunks arise from the same ventricle. The essence of these anomalies is that the interventricular communication is an integral part of the circulation. Unless the surgeon constructs an alternative route of exit, closure of this defect would isolate one of the ventricles. The usual surgical approach, therefore, is to patch the hole between the ventricles into one or other of the subarterial outflow tracts. This means that all the components of the ventricular outflow tracts are of potential surgical importance. In hearts with double outlet right ventricle, the VSD can be categorized as being subaortic, subpulmonary, doubly committed, or non-committed. It is also important to determine whether its anatomical borders, as seen from the right ventricle, are in part fibrous or exclusively muscular, so as to establish the location of the atrioventricular conduction axis. It is possible, according to the nature of these borders, to place the defects into one of three groups, perimenbranous, muscular, or doubly committed and juxta-arterial. The size of the defect is another important surgical consideration. Double outlet left ventricle is a significantly more rare malformation, but the rules for determining the disposition of the conducting tissues are the same.