The law describing the pressure–volume relationship of a gas at constant temperature is Boyle’s law. It states that pressure and volume are inversely proportional when the amount of gas and temperature remain unchanged.
The relationship is written PV = k, or, when comparing two states, P₁V₁ = P₂V₂. Thus, compressing a fixed sample of gas to half its volume doubles its pressure; doubling the volume halves the pressure. The pressure used in precise applications is absolute pressure.
Robert Boyle published his influential experiments in 1662, using a J-shaped tube and mercury to trap and compress air. French physicist Edme Mariotte independently described the same relationship in 1679, so the law is also called Mariotte’s law or the Boyle–Mariotte law in some countries.
Boyle’s law is an idealized rule: real gases depart from it at very high pressures or very low temperatures. It is also easy to confuse with Charles’s law, which links volume and temperature at constant pressure, or Gay-Lussac’s law, which links pressure and temperature at constant volume.