The heart emits a very regular set of weak electrocardiographic currents a split second before each beat, and they make up a very regular set of ECG patterns. In most cases, each set of ECG graphs appears sequentially as P waves, QRS wave clusters, J-points, ST segments, and T waves, and in a few patients, low U waves may also follow the T waves. There are also a number of time intervals between waveforms that are important in the ECG: namely, the ST segment, PR interval, PP interval, and RR interval.