A curve showing variation of volume of a substance taken along the X-axis and the variation of pressure taken along Y-axis is called an **indicator diagram or P-V diagram**. The shape of the indicator diagram shall depend on the nature of the thermodynamical process the system undergoes.

Let consider one mole of an ideal gas enclosed in a cylinder fitted with a perfectly frictionless piston. Let P_{1}, V_{1} and T be the initial state of the gas. If dV is an infinitesimally small increase in volume of the gas during which the pressure P is assumed to be constant, then small amount of workdone by the gas is, **dW = PdV**

In the indicator diagram, dW = area a_{1}b_{1}c_{1}d_{1}

The total workdone by the gas during expansion from V_{1} to V_{2} is

**W = ^{v2}∫_{v1} PdV = area ABCD**, in the indicator diagram.

Hence, in an indicator diagram the area under the curve represents the work done (Figure).