I. INTRODUCTION A clearer conception of the temporal relation that each variation of the electrocardiogram bears to systole and diastole of the auricles and ventricles is of twofold importance. It is essential, if the electrocardiogram is to be utilized, directly or indirectly, as a diagnostic criterion of the functional capacity of the heart. It is indispensable in explaining the ultimate cause of the electrocardiogram variations, for obviously no dynamic process can be held responsible for any electrical variation with which it is not isochronous. In discussing the time relations it is important to be oriented as to the precise events that merit comparison. It is quite generally recognized that the electrical variations recorded in the electrocardiogram by the three customary leads do not occur in an invariable relation to each other or to a definite phase of the cardiac cycle. Einthoven,1 in 1908, first directed attention to the fact that