In order to recognize and interpret the abnormal electrogram, it is first necessary to be familiar with the normal electrocardiogram and to appreciate the fact that the electrocardiographic waveform is influenced by the age, sex, race, and body build of the patient. In addition, the patient's position and the location of the electrodes on the body surface will influence the waveform observed. In this section, we will discuss the characteristics of the normal P wave, QRS complex, ST segment, T and U waves and will show how these components can be expressed as a series of spatial vectors having direction and magnitude. This approach, developed by Dr. Robert Grant in the 1950's, is of great help in defining the normal electrocardiographic waveform and in recognizing deviations from normal.