Millimetres of Mercury (mmHg)
Modern equivalent
133.32 Pascals
The height a column of mercury rises under a given pressure, and still the worldwide standard for reporting blood pressure.
Early sphygmomanometers were mercury columns, so a blood pressure reading was quite literally the height in millimetres that the mercury rose. Modern instruments contain no mercury at all, yet the unit was deliberately retained so that more than a century of clinical records remain directly comparable. It is all but identical to the torr, differing only in the fourth decimal place.