text: Traditional and presently most widespread categorisation of pure working fluids was first used by H. Tabor et al. and O. Badr et al. dating back to the 60s. This three-class classification system sorts pure working fluids into three categories. The base of the classification is the shape of the saturation vapour curve of the fluid in temperature-entropy plane. If the slope of the saturation vapour curve in all states is negative (ds/dT<0), which means that with decreasing saturation temperature the value of entropy increases, the fluid is called wet. If the slope of the saturation vapour curve of the fluid is mainly positive (regardless of a short negative slope somewhat below the critical point), which means that with decreasing saturation temperature the value of entropy also decreases (dT/ds>0), the fluid is dry. The third category is called isentropic, which means constant entropy and refers to those fluids that have a vertical saturation vapour curve (regardless of a short negative slope somewhat below the critical point) in temperature-entropy diagram. According to mathematical approach, it means a (negative) infinite slope (ds/dT=0). The terms of wet, dry and isentropic refer to the quality of vapour after the working fluid undergoes an isentropic (reversible adiabatic) expansion process from saturated vapour state. During an isentropic expansion process the working fluid always ends in the two-phase (also called wet) zone, if it is a wet-type fluid. If the fluid is of dry-type, the isentropic expansion necessarily ends in the superheated (also called dry) steam zone. If the working fluid is of isentropic-type, after an isentropic expansion process the fluid stays in saturated vapour state. The quality of vapour is a key factor in choosing steam turbine or expander for heat engines. See figure for better understanding.