It is known experimentally that for gases at low density (such that their molecules occupy a negligible fraction of the total volume) and at temperatures well above the boiling point, pressure is proportional to temperature.

This proportionality is the basis of the constant-volume gas thermometer. When temperature is held constant, either pressure or volume is proportional to the number of molecules.

When the proportionalities are combined into a single equation, the constant of proportionality is independent of the composition of the gas. The resulting equation for all gases applies in the limit of low density and high temperature. A gas at that limit is called an Ideal Gas; it obeys the Ideal Gas Law, which is also called the equation of state of an Ideal Gas.