Contraction of the atria and ventricles is tightly coordinated by a wave of depolarisation spreading through the muscular walls of these chambers. The depolarisation wave reflects movement of charge across cardiomyocyte membranes and is in effect an electrical current spreading through the heart. Following contraction, cardiac muscle returns to a resting state and this is associated with reversal of the movement of charge across the myocyte membranes, this second wave of electrical activity is termed cardiac repolarisation. The leads of the ECG machine are designed to detect and record these two waves of cardiac electrical activity. The depolarisation and repolarisation waves spread through the heart in a highly predictable pattern and to understand the ECG readout, the pattern of spread of cardiac electrical activity needs to be understood.