During normal atrial depolarization, the main electrical vector is directed from the SA node towards the AV node, and spreads from the right atrium to the left atrium. This turns into the<unk>  on the ECG, which is upright in II, III, and aVF (since the general electrical activity is going toward the positive electrode in those leads), and inverted in aVR (since it is going away from the positive electrode for that lead). The<unk>  morphology is best determined in leads II and V1 during sinus rhythm.
P wave
V wave
QRS complex
T wave
A wave
ST interval
PR interval
PP interval