Normal<unk>  In electrocardiography, the<unk>  represents the repolarization of the ventricles. The interval from the beginning of the QRS complex to the apex of the<unk>  is referred to as the absolute refractory period. The last half of the<unk>  is referred to as the relative refractory period or vulnerable period. The<unk>  contains more information than the QT interval. The<unk>  can be described by its symmetry, skewness, slope of ascending and descending limbs, amplitude and subintervals like the Tpeak–Tend interval. In most leads, the<unk>  is positive. This is due to the repolarization of the membrane. During ventricle contraction (QRS complex), the heart depolarizes. Repolarization of the ventricle happens in the opposite direction of depolarization and is negative current, signifying the relaxation of the cardiac muscle of the ventricles. This double negative of direction and charge is why the<unk>  is positive; although the cell becomes more negatively charged, the net effect is in the positive direction, and the ECG reports this as a positive spike. However, a negative<unk>  is normal in lead aVR. Lead V1 may have a<unk>  with positive, negative, or biphasic where positive is followed by negative, or vice versa. In addition, it is not uncommon to have an isolated negative<unk>  in lead III, aVL, or aVF. A periodic beat-to-beat variation in the amplitude or shape of the<unk>  may be termed<unk>  alternans.
T wave
P wave 
QRS complex
U wave
J wave
ST segment
Delta wave
PR interval