Einthoven's triangle: I, II, III

The three standard limb leads form Einthoven's triangle in the frontal plane. They are not independent: by construction III = II − I (Einthoven's law), which is why lead III is derived, not measured.

• Lead I (right → left arm): upright P/QRS/T; it measures leftward forces.
• Leads II and III look inferiorly (toward the feet) and are normally upright.
• Einthoven's law: III = II − I, the derived limb leads are correct by construction.