The state variables for gas under study are pressure, volume, temperature, and the number of moles. Avogadro's law states that the gas volume is proportional to its number of moles at constant temperature and pressure. Similarly, Boyle's law expresses that the pressure is inversely proportional to the volume of the gas for a given number of moles and at constant temperature. Furthermore, Charles's law gives the relationship between the volume and temperature of the gas at constant pressure and a given number of moles. Similarly, Gay-Lussac's Law states that the pressure of a gas is proportional to its temperature for a given number of moles and at constant volume. These four laws combined give the ideal gas equation. The proportionality constant is the universal gas constant, with a value of 8.314 joules per mole kelvin in the SI units. It is independent of the type of gas under the study. The ideal gas equation describes any gas at higher temperatures and low pressures.