The superior/inferior orientation of the septum will depend on the position of the heart in the chest. For this reason, the initial portion of the QRS complex will vary and this variability will be more prominent in the frontal plane, or limb leads (leads I-aVF), than in the horizontal plane, or chest leads (leads V1-V6) because the frontal plane leads are more reflective of the heart's superior/inferior spatial orientation. This is demonstrated in the next several pages.