Holloman and Jaffe determined the value of "C" experimentally by plotting hardness versus tempering time for a series of tempering temperatures of interest and interpolating the data to obtain the time necessary to yield a number of different hardness values. This work was based on six different heats of plain carbon steels with carbon contents varying from 0.35%–1.15%. The value of "C" was found to vary somewhat for different steels and decrease linearly with the carbon content of a steel grade. Holloman and Jaffe proposed that "C" = 19.5 for carbon and alloy steels with carbon contents of 0.25%–0.4%; and "C" = 15 for tool steels with carbon contents of 0.9%–1.2%.