Fig. 1: A graph of pressure, in units of 10 to the 5 pascal, against volume in litres. A rectangle has corners A at 2.0 litres and 1.0, D at 2.0 litres and 3.0, C at 5.0 litres and 3.0, and B at 5.0 litres and 1.0. Arrows show the gas going A to D (up), D to C (right), C to B (down) and B back to A (left): clockwise.