The Joule–Thomson coefficient, μ μ J = ∂ ∂ p T | h , is of practical importance because the two end states of a throttling process ( h 2 = h 1 ) lie on a constant enthalpy curve. Although ideal gases, for which h = h ( T ) , do not change temperature in such a process, real gases do, and it is important in applications to know whether they heat up or cool down.