Einthoven's triangle

[2] Einthoven used these measuring points, by immersing the hands and feet in pails of salt water, as the contacts for his string galvanometer, the first practical ECG machine.

[5] The right leg electrode acts to reduce interference, and can be placed anywhere without an effect on the ECG results.

[6] 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.

[7] Einthoven's triangle can be helpful in the identification in incorrect placement of leads.