A statistical relationship between the period of a pulsating variable and its luminosity; also known as the Leavitt law after its discoverer, H. S. Leavitt. For classical Cepheids and W Virginis stars, the relationship is approximately linear: the longer the period, the greater the luminosity. Classical Cepheids show a greater colour dependence, so it is preferable to use a period–luminosity–colour relation. Mira stars have a non-linear PL relation, but it is well-established and useful for determining distances within the Galaxy. The Cepheid PL relation is the basis of the traditional method of determining distances to nearby galaxies.