1. Temperature - Entropy plot
Since , the area under a curve drawn on temperature – entropy plot (with temperature on Y-axis and entropy on X-axis) is equal to the heat transferred in the process.
Reversible adiabatic process on T–S plot
Reversible isothermal process on T–S plot
Since Qrev = 0 for any adiabatic process, dS = 0 and hence S = constant. So, a reversible adiabatic process is also called isentropic process.

2. Carnot cycle on T–S plot

3. Clausius inequality Efficiency of a general cycle is equal to
Efficiency of a reversible cycle is equal to
But, from Carnot theorem,

4. Clausius inequality But, for a reversible cycle,
(for any process AB reversible or irreversible)
(or)

5. Clausius inequality For a reversible process, For any process AB,
For any cycle,
But, cyclic integral of a property is always equal to 0
(for a reversible process)
(for an irreversible process)
(the cycle is impossible)