  We analyze the relations among central gas density, core radius, and
temperature of X-ray clusters by plotting the observational data in the
three-dimensional ($\log \rho_0$, $\log R$, and $\log T$) space and find that
the data lie on a 'fundamental plane'. Its existence implies that the clusters
form a two-parameter family. The data on the plane still has a correlation and
form a band on the plane. The observed relation $L_{\rm X} \propto T^3$ turns
out to be the cross section of the band perpendicular to the major axis, while
the major axis is found to describe the virial density. We discuss implications
of this two-parameter family nature of X-ray clusters.
