Conventional millimetre of mercury

One way of defining pressure is in terms of the height of a column of fluid that may be supported by that pressure; or the height of a column of fluid that exerts that pressure at its base if you like. Although a manometer may use any fluid in principle, common fluids like water give heights that can't be contained in a normal room- water is of the order of 20 metres to give atmospheric pressure. So we want a very dense fluid- mercury. Normal atmospheric pressure can support around 760mm of mercury; hence 1/760th of an atmosphere, or 1 mm of mercury, has been a convenient measure of pressure for a long time.