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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.