Summary A simple means of cross-checking the vectorcardiographic loop-directions by the waveforms of electrocardiographic leads is described in this paper. The QR or RS segment shifts its phase within a QRS complex as an observer glances stroboscopically through the electrocardiograms of a coordinate plane. The inscription of the QRS loop on this plane by the terminal point of the “cardiac vector” must be made in a rotational direction the same as the phase progression of the QR or RS in leads around the origin. This method, having been checked with published cases, proves to be satisfactory.