According to Einthoven's law:
The sum of the voltages in lead 1&3 equals the voltage in lead 2, mathematically this principle is called as Einthoven law.
Einthoven’s Law:
Einthoven’s law states that if the ECGs are recorded simultaneously with the three limb leads, the sum of the potentials recorded in leads I and III will equal the potential in lead II.
1+2=3
1+3=2
2+3=1
1+2+3=0