  Willem Einthoven is widely considered the father of the electrocardiogram
(ECG). In 1912, he proposed a method of determining the electric axis of the
heart by using an imaginary equilateral triangle connecting the limb leads, now
known as Einthoven's triangle. In 1924, Einthoven was awarded the Nobel Prize
in Physiology or Medicine for his discovery of the mechanisms of the
electrocardiogram. More than a century later, Einthoven's triangle is still at
the heart of ECG interpretation. It defines the axes of the ECG leads in the
frontal plane, that in turn, determines the axis of the cardiac electric
dipole. The method is ubiquitously taught in lectures and applied in clinical
settings. But Einthoven did not provide a proof for choosing the equilateral
triangle. Future medical literature have not explored its origins. This paper
provides a formal proof of its derivation to complete this important chapter in
medical history and medical education. In addition, the proof determines the
geometric conditions for alternative systems of bipolar ECG lead configurations
in the frontal plane.
