The same three
leads that form the standard leads also form the three unipolar leads known
as the augmented leads. These three leads are referred to as aVR
(right arm), aVL (left arm) and aVF
(left leg) and also record a change in electric potential in the frontal
plane. These leads are unipolar in that they measure the electric potential at one point with respect to a null point (one which doesn't register any significant variation in electric potential during contraction of the heart).
This null point is obtained for each lead by adding the potential from the other two leads. For example, in lead aVR, the electric potential of the right arm is compared to a null point which is obtained by adding together the potential of lead aVL and lead aVF.