Einthoven's law
Einthoven's Law states that if the electrical potentials of any two of the three bipolar limb electrocardiographic leads are known at any given instant, the third one can be determined mathematically by simply summing the first two (but note that the positive and negative signs of the different leads must be observed when making this summation). Thus the sum of the voltages in leads I and III equals the voltage in lead II.
I + III = II
I – III = II
I + II + III = 0
I + III = avL