Each beat of the heart requires an electrical impulse to trigger it. Normally, electrical impulses originate from the sinus node of the heart, called sinus rhythm, which is the normal heart rhythm. The atria or ventricles can also emit electrical impulses. If they emit electrical impulses that precede the normal sinus node impulses, a preterm contraction, or premature beat, can occur, which can be atrial, junctional, or ventricular. If a relationship is formed between this premature beat and the normal sinus impulse, it becomes a conjunctive rhythm. If a sinus beat is followed by a preterm contraction, followed by another sinus beat and a preterm contraction, this is a duplex rhythm. If two normal beats are followed by a pre-term contraction, this is a triplet rhythm.