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Challenging new concepts of the intraventricular conduction system of the heart have revitalized interest in electrocardiography and have increased its diagnostic yield as nothing else since the precordial and vectorial analysis Frank N. Wilson's group did a quarter of a century ago. Modern electrocardiographers are as obligated to recognize hemiblock of this conduction system as their predecessors were obligated to recognize bundle-branch block and infarction of the myocardium. Beyond this, they must take on the new responsibility to make this new diagnostic information meaningful and useful in the practice of the clinician who depends upon their interpretation of the electrocardiogram. They need to show how hemiblock can simulate infarction when it is actually absent or how it may conceal infarction when it is actually present. They must make the clinician aware of those circumstances in which multiple blockade within the fascicles portends life-threatening disturbances of rhythm in time to take