  Let $(W, S)$ be a Coxeter system. We give necessary and sufficient conditions
on the Coxeter diagram of $(W, S)$ for $W$ to be relatively hyperbolic with
respect to a collection of finitely generated subgroups. The peripheral
subgroups are necessarily parabolic subgroups (in the sense of Coxeter group
theory). As an application, we present a criterion for the maximal flats of the
Davis complex of $(W,S)$ to be isolated. If this is the case, then the maximal
affine sub-buildings of any building of type $(W,S)$ are isolated.
