The first electrical activity conspicuous in sinus rhythm is depolarization of the atria; a P wave occurs. The beginning of the P wave is indicated by a solid arrow () and in the diagrams by a solid line (\).

In sinus rhythm the P wave has the largest positive deflection in lead II (the atrial vector runs parallel to lead II in sinus rhythm).

The occurrence of supraventricular extrasystoles (SVES) is indicated by a dot with two arrows () and always occurs midway at atrial level.