Einthoven's triangle

Einthoven's triangle is an imaginary formation of three limb leads in a triangle used in electrocardiography, formed by the two shoulders and the pubis. The shape forms an inverted equilateral triangle with the heart at the center that produces zero potential when the voltages are summed. It is named after Willem Einthoven, who theorized its existence.
Einthoven used these measuring points, by immersing the hands and foot in pails of salt water, as the contacts for his string galvanometer, the first practical ECG machine.

Electrodes may be placed distally or proximally on the limb without affecting the recording. The leg electrode acts as a grounding lead, and either the right or left leg can be used as a grounding lead without an effect on the ECG results.
Each lead measures the electric field created by the heart during the depolarization and repolarization of myocytes. The electric field can be represented as a vector that changes continuously and can be measured by recording the voltage difference between electrodes.

Though Einthoven's triangle is no longer used in contemporary ECGs, Einthoven's triangle can be helpful in the identification in incorrect placement of leads. (Incorrect placement of leads can lead to error in the recording, which can ultimately lead to misdiagnosis.) If the arm electrodes are reversed, lead I changes polarity, causing lead II and lead III to switch. If the right arm electrode is reversed with the leg's electrode, lead II changes polarity, causing lead I to become lead III, and vice versa. Reversal of the left arm and leg causes a change in polarity of lead III and switching of leads I and II.