Bipolar limb lead system

Einthoven described a system of three bipolar leads located at the right arm, left arm, and left leg to form a triangle. Lead I represents the potential difference between the right and left arm; an electrical impulse moving from right to left generates a positive ECG deflection in this lead. Lead II is the potential difference between the right arm and leg; a positive ECG deflection occurs when the impulse direction is toward the leg. Lead III is the potential difference between the left arm and leg; there is a positive ECG deflection when the impulse direction is toward the leg.