The mechanism of PVC’s may reflect enhanced automaticity or reentry, either of which can produce uniform or multiform PVC’s. PVC's arise in the ventricles and are wide and different from the patient’s sinus QRS’s and are not preceded by premature P waves. The PVC’s morphology may be uniform (unchanging QRS morphology) or multiform (varying QRS morphologies). If uniform, the coupling interval (the time between the conducted beat and the PVC) may be constant. If multiform, the coupling interval typically varies. Since PVC’s reflect abnormal depolarization, the ST-T configuration (repolarization) is also different from the sinus beats. The coupling interval can vary from very early (e.g. a PVC on top of a T wave) to late, allowing the next expected sinus P wave to be visible prior to the PVC. The significance of PVC’s is determined by the clinical setting in which they occur. The following animation illustrates what we have just described.