The electrical activity that accompanies the cardiac cycle
can easily be measured and used diagnostically. The flow of
positive electrical
charges can be measured and tracked with
strategically placed electrodes attached to the surface of the
skin. There are at least 12 different lead pairs or positions for
measurement on the body's surface: six limb leads;
I, II, III,
aVR, aVF and aVL, and six chest leads;
V1 - V6.
A wave of 
depolarization advancing toward a positive skin
electrode will cause an upward deflection on an ECG strip. The
degree of deflection is related to the thickness or length of
muscle through which the positive charge travels. If the
depolarization wave moves in the opposite direction away from the
positive electrode, the ECG deflection will be below the
baseline.
ECG's are used to assess a number of items including: rate, rhythm, heart axis, heart muscle hypertrophy and evidence of ischemia or infarction.
Lead I consists
of a
positive electrode attached to the left
arm or shoulder and a negative one on the right arm or shoulder.
A wave of depolarization on the heart that advances toward the
positive lead causes a positive deflection on the ECG strip. If
we recall the position of the heart in the chest, we would expect
to see a moderate 
upward
deflected P-wave for atrial
depolarization and also for the R-wave for ventricular
depolarization.
Lead II has its
positive electrode at the left leg or lower
left chest and its negative electrode at the right arm or
shoulder. This pair is more in line with the long axis of the
heart, thus the upward deflections are greater than in Lead I.
Lead III has its
positive electrode at the lower left leg or
lower left chest and the negative electrode at the upper left arm
or shoulder. The positive deflections of the P- and R-waves are
minimal. The leads form a 
triangle with 60ø at each internal
angle. If the sides of the triangle were moved inwardly to bisect
the location of the AV node the lines would have external angles
of 60ø.