Electrocardiograms are recorded based on the input from leads connected to the body at specific points. Einthoven proposed that the heart is an electric generator that acts approximately as a dipole; that is, as a pair of electrodes (electrical terminals) separated by a short distance. He regarded the electrical actions of the heart as quantifiable vectors consisting of a magnitude and a direction, and proposed an analytic scheme in which three vectors form a triangle, called today the Einthoven triangle.

The term “lead” has two meanings here. It can refer to an actual electrode and cable connected to the electrocardiograph. It also refers specifically in electrocardiography to the voltage difference between two or more of these electrodes. In the simplest case a lead involves two electrodes attached to the skin. The leads used by Einthoven, called limb leads, were obtained from pairs of electrodes attached at the left leg (Lead 1), the left arm (Lead 2), and the right arm (Lead 3). Einthoven recognized that these voltages were not independent, but that Lead I + Lead III = Lead II, a relationship known today as Einthoven’s law.