The vector of atrial depolarization reflects the spread and is directed first inferiorly and then to the left. Because the right atrium is depolarized before the left atrium, the initial portion of the depolarization vector is directed anteriorly, while its terminal portion is directed posteriorly. Normally about 120 msec (0.12 seconds) is required to depolarize the atria. These characteristics of atrial depolarization, i.e. the sequence, vectorial direction and duration, are reflected in the shape and duration of the P wave on the 12 lead body surface electrocardiogram.