ECG tracings are recorded on grid paper. The horizontal axis of the
EKG paper records time, with black marks at the top indicating 3 second intervals.
Each second is marked by 5 large grid blocks. Thus each large block equals 0.2 second. The vertical axis records EKG amplitude (voltage). Two large blocks equal 1 millivolt (mV). Each small block equals 0.1 mV.
Within the large blocks are 5 small blocks, each representing 0.04 seconds.