  This definition, of thermodynamic entropy, depends upon cyclic processes that can reach the equality, which are called reversible processes. The existence of such processes between thermodynamic states allows the entropy differences between those states to be determined and, by extension to all states, defines a thermodynamic entropy function that is globally unique (up to a rescaling S′ = a−1 S + b, where a and b are constants, and a is the multiplicative constant from the temperature scale). It may be noted that if there exist states that cannot be connected by a reversible process, it is still always possible to define an entropy function that satisfies