The ECGs shown here are from the same patient recorded before (left) and after (right) the development of RBBB. The initial part of the QRS complex, i.e. that associated with septal depolarization, is unaltered because the septum receives its activation from the left bundle, The abnormality in depolarization caused by RBBB is marked with the red arrows. It causes the terminal S wave in leads I, V5, and V6 and by the R’ in V1. The spatial vector of this, the terminal portion of the QRS complex, is directed anteriorly and to the right.