The two secondary characteristic points, namely M and N are defined as local entropy extrema on the saturation vapour curve, more accurately, at those points, where with the decrease of the saturation temperature entropy stays constant: d s /d T =0. We can easily realise that considering traditional classification, wet-type fluids have only primary (A, C and Z), dry-type fluids have primary points and exactly one secondary point (M) and redefined isentropic-type fluids have both primary and secondary points (M and N) as well. See figure for better understanding.