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Here are three enlargements at differing magnifications of the same EKG strip.
This is standard pre-printed EKG paper.  On it is a recording made at the
standard speed of 25 mm (or almost exactly one inch) per second.  Note that
the paper is divided into small and large boxes by light and heavy lines.
Horizontally, the light lines mark intervals of 0.04 seconds, and the heavy
lines mark intervals of five times that: 0.2 seconds.  So, five large boxes
represent a duration of one second.  Similarly, three small boxes represent
a duration of 0.12 seconds.  The vertical scale represents voltage, with each
light line corresponding to 0.1 millivolt.  However, this scale is reliable only
on 12-lead EKG machines, as it is often altered on other monitors for ease
of observation.  Also note the marks in the bottom margin:  they represent
intervals of three seconds.  Two of these intervals are a tenth of a minute,
so the number of beats between two of these marks, multiplied by ten,
give the approximate heart rate.





On this recording, the EKG and paper markings are printed together at the
standard speed of 25 mm (or almost exactly one inch) per second.  Note that
the paper is divided by light and heavy dots to mark intervals of time in the
same increments as pre-printed paper. The lines formed by light dots mark
intervals of 0.04 seconds, and the lines of heavy dots mark intervals of five
times that: 0.2 seconds. So, five large boxes represent a duration of one
second.  Similarly, three small boxes represent a duration of 0.12 seconds.
The vertical scale represents voltage, with a calibration marker showing one
millivolt printed near the right end of the strip.  The marks in the top margin
(not seen here) represent intervals of three seconds.  Two of these intervals
are a tenth of a minute, so the number of beats between two of these marks,
multiplied by ten, give the approximate heart rate.