An electrocardiographic lead is a recording electrode or a pair of recording electrodes at a specified location.
In clinical practice, twelve leads are usually used in the diagnostic EKG, although there is no limitation to the number of leads one may select for special purposes.
The leads are usually placed on the wrists and ankles but since the limbs serve as electrical cables, the electrodes behave as if they were at the shoulders and groin.
A right leg electrode is used as the ground.
