The localization of the origin of ventricular extrasystoles and of the lesion in bundle branch block is still subject to dispute. Three separate ideas prevail: The classic interpretation based on the fundamental studies of Lewis1and of Rothberger and Winterberg2is that the main ventricular deflection in left ventricular extrasystoles and in right bundle branch block is upright in lead 1 and inverted in lead 3. Right ventricular extrasystoles and left bundle branch block have an opposite configuration. This conception is based on experimental work performed on dogs. The new terminology based on the observations of Barker, MacLeod and Alexander,3Oppenheimer and his collaborators4and Wilson5is exactly the opposite of the classic interpretation. This conception has been accepted by most but not by all authorities in this field. Finally there is the opinion of Katz6that localization of extrasystoles and bundle