Here I introduce the T-s diagram for ideal gasses. I show that an isentropic expansion is a vertical upward line on this and an isentropic compression is a vertical downward line. I then show that in a real process, entropy increases and the real lines end up to the right of the ideal ones. This menas that we actually get less energy out of an expansion and have to put more in for a compression. I then show what these mean in terms of isentropic efficiencies.