The electrocardiogram is a photographic record of the differences in electrical potential existing between two points on the body surface at every instant during the time the record is being taken. On the ordinates of the record one reads the potential differences; on the abscissas the time is given. When the electrocardiogram in the three standard limb leads is made, measurement of the potential differences at any instant in any two leads supplies data from which one may calculate the apparent direction and magnitude of the electromotive force at that instant as that force is projected on the frontal plane of the body. The direction of the force at each instant is the instantaneous electrical axis. Since the electromotive force has both direction and magnitude, it is a vector quantity. One may determine the magnitudes and directions of the vectors for each instant of the ventricular (or auricular) electrocardiogram and