Systems for describing congenital cardiac malformations have frequently been based upon embryological concepts and theories. As useful as these systems have been, they have often had the effect of confusing the clinician, rather than clarifying the basic anatomy of a given lesion. As far as the surgeon is concerned, the essence of a particular malformation lies not in its presumed morphogenesis, but in the underlying anatomy. An effective system for describing this anatomy must be based upon the morphology as it is observed. At the same time, it must be capable of accounting for all congenital cardiac conditions, even those that, as yet, might not have been encountered. To be useful clinically, the system must be not only broad and accurate, but also clear and consistent. The terminology used, therefore, should be unambiguous. It should be as simple as possible. The sequential segmental approach provides such a system, particularly when the emphasis is placed on its surgical applications. The basis of the system is, in the first instance, to analyse individually the architectural make-up of the atrial chambers, the ventricular mass, and the arterial segment. Emphasis is thus given to the nature of the junctional arrangements (Figure 6.1). Still further attention is devoted to the interrelationships of the cardiac structures within each of the individual segments. This provides the basic framework within which all other associated malformations can be catalogued.