The basic structure of a Carnot cycle. In Step 1, there is an isothermal reversible expansion at the temperature Th. Step 2 is a reversible adiabatic expansion in which the temperature falls from Th toTc. In Step 3 there is an isothermal reversible compression at Tc, and that isothermal step is followed by an adiabatic reversible compression, which restores the system to its initial state.