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Here is a recording of a normal beat, with important elements labeled.  The first element is the P-wave, which is a recording of the electrical activity of the atria.  The larger deflection, showing larger voltages caused by the electrical activity of larger muscle masses, is called the QRS complex, and is a recording of ventricular depolarization.   Then follows the T-wave, a recording of ventricular repolarization.


Here is the same beat with other important segments labeled.
The PR interval is the time elapsed during atrial depolarization and A-V nodal delay.   Here it lasts about three and three-quarter small boxes, or 3.75 x 0.04 = 0.15 seconds.
The QRS duration is the time during which the ventricles depolarize.   Here it lasts about one and one-half small boxes, or 1.5 x 0.04 = 0.06 seconds.
The QT interval is the time during which the ventricles depolarize and repolarize.  Here that interval is two small boxes, then a big box, then three more small boxes, or 2 x 0.04 plus 0.2 plus 3 x 0.04 = 0.40 seconds.  The ST segment is not clearly demarcated, but is very important for evaluating certain disease states.