What does the small grid in the ecg paper represent

There are interweaved lines on the ecg paper, which form many very standard small grids. The two vertical lines of each small grid are 1 mm apart, representing a time of 0.04 s; the two horizontal lines of each small grid are also 1 mm apart, representing a voltage of 0.1 MV. The paper speed of electrocardiography must be 25 mm per second, and the voltage calibration must be 10 cells. If the paper speed is 50 mm / s, then each small cell represents 0.02 s; if the voltage calibration is halved to 5 small cells, then each small cell represents 0.2 MV, but this is used under special circumstances.