The decimal reduction time D is defined as the time for a tenfold reduction in the number of survivors of a given micro-organism, in other words the time for one log cycle reduction in the microbial population. Higher values of D imply, at a given temperature, greater resistance of micro-organisms to thermal death. Because D depends upon temperature, the temperature in °C is appended as a subscript. Thus D121.1 is the time required at 121.1 °C to reduce a microbial population by 90%. A temperature of 121.1 °C (or 250°F) is used as a common reference point and therefore, because of its importance, this is sometimes referred to as Do.