Combining the laws of Charles, Boyle and Gay-Lussac gives the combined gas law, which takes the same functional form as the ideal gas law says that the number of moles is unspecified, and the ratio of  to  is simply taken as a constant:
where  is the pressure of the gas,  is the volume of the gas,  is the absolute temperature of the gas, and  is a constant. When comparing the same substance under two different sets of conditions, the law can be written as