http://en.wikipedia.org/wiki/Van_der_Waals_equation (the graph of Maxwell equal area rule) When the temperature is under the critical temperature, then at certain pressure, there will be three possible point for the volume of the system. What does that mean? Do they mean the volume of the system will change between that 3 volume? From the above link, they say they will be phase for liquid and gas, what do they mean? Thank you.