Several instruments can measure blood pressure quickly and with little discomfort. A sphygmomanometer is commonly used. It consists of a soft rubber cuff connected to a rubber bulb that is used to inflate the cuff and a meter that registers the pressure of the cuff. The meter may be a dial or a glass column filled with mercury. Blood pressure is measured in millimeters of mercury (mm Hg) because the first instrument used to measure it was a mercury column. When a sphygmomanometer is used, a person sits with legs uncrossed and back supported. An arm is bared (if a sleeve is rolled up, caution is needed to ensure that it is not tight around the arm), bent, and resting on a table, so that the arm is about the same level as the heart. The cuff is wrapped around the arm. Using a cuff that is proportional to the size of the arm is important. If the cuff is too small, the blood pressure reading is too high. If the cuff is too large, the reading is too low. Listening with a stethoscope placed over the artery below the cuff, a health care practitioner inflates the cuff by squeezing the bulb until the cuff compresses the artery tightly enough to temporarily stop blood flow, usually to a pressure that is about 30 mm Hg higher than the person's usual systolic pressure (the pressure exerted when the heart beats). Then the cuff is gradually deflated. The pressure at which the practitioner first hears a pulse in the artery is the systolic pressure. The cuff continues to be deflated, and at some point, the sound of blood flowing stops. The pressure at this point is the diastolic pressure (the pressure exerted when the heart relaxes, between beats). Some instruments can measure blood pressure automatically, without use of a stethoscope or rubber bulb. These devices may fit around the upper arm, finger, or wrist. For people older than 50, blood pressure measured at the upper arm is the most accurate. Sometimes a precise measurement of blood pressure is needed—for example, for a person in an intensive care unit. In such cases, a catheter can be inserted inside an artery to measure blood pressure directly. Instruments to measure blood pressure are available for home use by people who have high blood pressure.