The paper on which the ECG is drawn is divided up into 1 millimeter lines horizontally and vertically. The vertical lines represent passage of time. Because the paper moves at a rate of 25 mm per second, each 1 mm line represents 0.04 seconds of time. Every fifth line is darkened to help with counting. The vertical direction represents the strength of electrical voltage.
Positive voltage moves the stylus up, negative voltage moves it downward.
Each millimeter vertically represents 0.1 millivolt. Ten vertical boxes is one millivolt. The horizontal direction represents passage of time. Look for P waves. Check the PR interval.