The initial vector (1) reflects the small Q in leads I and aVL and the small R wave in II, III, and aVF. It is directed inferiorly with an axis of + 90 degrees. The middle, or main QRS vector (2) reflects the R waves in leads l and aVL and the S waves in leads II,III and aVF. It is directed superiorly and to the left with an axis of -50 degrees. These two vectors are characteristic of left anterior fascicular block. The terminal vector (3) reflects the broad S wave in I and the broad R’ in aVR. It is directed to the right and slightly superiorly with an axis of +200 degrees and is characteristic of right bundle branch block.