The P wave and PsÊ-loop were derived from coplanar outlines of the contour of seven human atria, including left atrial enlargement, in which it was assumed that the process of depolarization spread in a radial fashion from the sinoatrial node. The "derived" planar projections of the P waves of the standard leads and PsÊ-loops were similar in relative amplitude and general contour to those recorded electrocardiographically and vectorcardiographically prior to death.