This brings us to Eintoven's Triangle. It's just a way of representing leads I, II and III (which were the first to be used) in a triangular arrangement so that they are easy to remember. I prefer the leads in a "compass" arrangement, as below, with their center points touching. It makes it easier in the future when the other leads are added...

aVF, aVR and aVL are called the unipolar limb leads. By calculation from the leads I, II and III, the EKG machine can display a voltage _as if_ there were a lead pointing from the center of the heart's electrical field to the left arm, the right arm and the feet. These leads are designated unipolar, because there really is no negative electrode - the signal is really just calculated. What this achieves for us is a way to look at potentials pointing in the half-way spots in between I, II and III. This gives an EKG more precision and diagnostic potential.