Normally, blood pressure is measured at the brachial artery of the right upper arm, and the left side can be chosen when the right side is not convenient for measurement or when the right forearm is missing. Because blood pressure is a pressure value, there will be a slight difference between different parts of the measurement. Normal people often use their right hand, so the measurement result of the right side is relatively higher and the left side is relatively lower, but the normal value will not exceed 20 mmHg. If the difference in blood pressure between the left and right arms exceeds 20 mmHg, it may be caused by a disease, such as aortic coarctation. Most of the reasons for choosing the right hand side now are due to clinical testing habits, as the standard standing position of the doctor or nurse is on the right side of the patient during the room visit, so the right arm is often chosen to measure blood pressure.