In a normal electrocardiogram (ECG or EKG),

B: The QRS complex consists of three individual waves: the Q, R, and S waves. The QRS complex results from depolarization of the ventricles, and the beginning of the QRS complex precedes ventricular contraction. A: The P wave results from depolarization of the atrial myocardium, and the beginning of the P wave precedes the onset of atrial contraction. C: The T wave represents the repolarization of the ventricles, and the beginning of the T wave precedes ventricular relaxation. D: During the P-R interval, the atria contract and begin to relax.