Grid lines and standardization of the ECG

The electrocardiogam is recorded on paper that has large boxes (heavy lines) of 0.5 cm sides. On the horizontal axis, each large box, which represents 0.2 seconds at a typical paper speed of 25mm/sec, is divided into five smaller boxes, each one representing 0.04 seconds. On the vertical axis, the large box also has five subdivisions, each 1 mm in height; 10 mm equals 1 mV with standard calibration.