The ratio of the value of P V to that expected for an ideal gas at the same temperature is called the compressibility factor, Z . For a gas, this is typically less than unity at low temperature and greater than unity at high temperature (see the discussion in compressibility factor). At low pressure, the value of Z always moves towards unity as a gas expands. Thus at low temperature, Z and P V will increase as the gas expands, resulting in a positive Joule–Thomson coefficient. At high temperature, Z and P V decrease as the gas expands; if the decrease is large enough, the Joule–Thomson coefficient will be negative.