The ideal gas law is the equation of state of a hypothetical ideal gas. The ideal gas law is a combination of two previous theories, Boyle’s law and Charles’ law. The former describes how the pressure of a gas decreases as the volume increases, meaning the two are inversely proportional. Charles’ law describes how gases expand when heated.
The two laws were combined by Emile Clapeyron in 1834 to create the ideal gas law: pressure x volume = moles of substance x constant x temperature, more commonly written as PV = nRT. Although limited, the law acts as an approximation to the behaviour of gases under many different conditions.