In 1913, Einthoven, Fahr, and de Waartr published a method of estimating the direction and manifest magnitude, at a given instant in the cardiac cycle, of that component of the heart’s electromotive force which is parallel to the plane defined by the standard limb leads. They utilized this method to study and to explain the modifications of the electrocardiographic deflections of these leads produced by respiratory variations in the position of the heart. The chief purpose which they had in mind seems to have been to find a way of distinguishing electrocardiographic phenomena due to extrinsic causes of this sort from those originating within the heart itself. More than thirty years have passed since this fundamental paper by Einthoven and his associates was written. No other has had so great an effect upon the development of our knowledge of the electrocardiogram; none has been the source of more inspiration; and none has been the subject of so much misunderstanding, so much critical examination, and so much controversial discussion. Why after all these years should there still be a wide difference of opinion regarding the correctness of the views expressed in this paper?