A frictionless turbine would expand the steam at constant entropy and land at a specific point on the chart. A real one generates entropy, so it lands to the RIGHT of that point — higher enthalpy, less work extracted. Comparing the two is the isentropic efficiency: $\eta_{isen} = \dfrac{h_1 - h_2}{h_1 - h_{2s}}$, read aloud eta-isentropic equals h-one minus h-two, over h-one minus h-two-s. The Greek letter $\eta$ is eta, and it means efficiency throughout thermodynamics.

Three enthalpies, all in kJ/kg, and one of them imaginary. $h_1$ is the inlet. $h_2$ is the actual exhaust, the one you can measure. $h_{2s}$ is the isentropic exhaust — the subscript s stands for the constant ENTROPY that got you there, and it is derived from the chart, never from a thermowell. Because the ideal machine falls further, $h_{2s}$ is always the LOWEST of the three.

Actual on top, ideal underneath, always. Turbines run 0.7 to 0.9, so an answer above 1 is not a triumph — it is a swapped ratio, every time. Nothing beats a frictionless expansion between the same two pressures; that is what makes it the yardstick.