aVR lead, "a-augmented,
V-voltage, R-right arm" lead. 
By changing settings on the ECG machine, we can make the
negative electrode on the right arm become the single existing
positive electrode, and make the negative electrodes consist of
both the left arm and left leg leads together. In this
configuration the flow of positive charge is at an angle between
Leads I and II and is referred to as the aVR lead, "a-augmented,
V-voltage, R-right arm" lead.
In contrast, the 
aVL
lead has its positive electrode at the
left arm. Together with its two negatives, right arm and left
foot, its reading will result in a vector bisecting limb Leads I
and III.
The 
aVF lead has
its positive electrode at the left foot or
lower left chest and its negative electrodes at the shoulders.
If the 
triangle
produced by leads aVR, aVL and aVF had its
sides collapsed to intersect at the AV node, we would also have
external angles of 60ø as seen in leads I, II and III. In
addition, if all limb lead vectors were superimposed, it would be
possible to study electrical activity from 6 
different directions
30ø apart from the frontal view.