A simple system for the analysis and classification of the univentricular heart is described. The basis of this approach is the use of a step by step sequential nomenclature with attention paid to the morphology, connexions and relationships of each cardiac segment in turn. Three major morphological types of univentricular heart are identified and these may be distinguished, by their characteristic trabecular patterns, into hearts of left ventricular type, right ventricular type and indeterminate type. With the exception of the last it is usual for a rudimentary second chamber to be present upon the use of a clear definition of "ventricle", depending on readily ascertainable anatomical characteristics. Each morphological type of univentricular heart may be associated with a wide variety of different connexions and intracardiac relationships and it is neither possible nor desirable to describe these in terms of a rigidly structured classification. Adherence to the simple semantic framework outlined would resolve much of the confusion and controversy which exists in relation to the univentricular heart.