The fundamental plane is a set of bivariate correlations connecting some of the properties of normal elliptical galaxies. Some correlations have been empirically shown.
The fundamental plane is usually expressed as a relationship between the effective radius, average surface brightness and central velocity dispersion of normal elliptical galaxies. Any one of the three parameters may be estimated from the other two, as together they describe a plane that falls within their more general three-dimensional space. Properties correlated also include: color, density (of luminosity, mass, or phase space), luminosity, mass, metallicity, and, to a lesser degree, the shape of their radial surface brightness profiles.