ECG signal is a kind of differential signal measured on two different parts of the human body. Therefore, two electrodes are necessary to obtain one channel of ECG signal. Then, operators should place electrodes on two different parts of the human body and connect them to the positive and negative pole of the electrocardiograph through lead wires. This method of connection to record the electrocardiogram is called the lead of the electrocardiogram. An internationally accepted lead system is the standard 12-lead system, which has three channels of standard limb leads--I, II, and III, three channels of augmented unipolar limb leads--aVR, aVL, and aVF, and six channels of chest leads--V1 ~ V6. At this time, a total of 10 electrodes (one for left arm, one for right arm, one for left leg, one for right leg and six for chest) need to be placed on the surface of human body. Some of the electrodes can be reused, so it is not necessary to use 24 electrodes. Generally, in the case of normal use, one or more channels of the standard 12 leads are selected for measurement. Measuring one lead occupies one channel of the sensor.