text: A PV diagram plots the change in pressure P with respect to volume V for some process or processes.  Typically in thermodynamics, the set of processes forms a cycle, so that upon completion of the cycle there has been no net change in state of the system; i.e. the device returns to the starting pressure and volume.
  The figure shows the features of an idealized PV diagram. It shows a series of numbered states (1 through 4).  The path between each state consists of some process (A through D) which alters the pressure or volume of the system (or both).
  A key feature of the diagram is that the amount of energy expended or received by the system as work can be measured because the net work is represented by the area enclosed by the four lines.
  In the figure, the processes 1-2-3 produce a work output, but processes from 3-4-1 require a smaller energy input to return to the starting position / state; so the net work is the difference between the two.
  This figure is highly idealized, in so far as all the lines are straight and the corners are right angles. A diagram showing the changes in pressure and volume in a real device will show a more complex shape enclosing the work cycle.  ().