  Let $(W,S)$ be a Coxeter system, and let $X$ be a subset of $S$. The subgroup
of $W$ generated by $X$ is denoted by $W_X$ and is called a parabolic subgroup.
We give the precise definition of the commensurator of a subgroup in a group.
In particular, the commensurator of $W_X$ in $W$ is the subgroup of $w$ in $W$
such that $wW_Xw^{-1}\cap W_X$ has finite index in both $W_X$ and $wW_Xw^{-1}$.
The subgroup $W_X$ can be decomposed in the form $W_X = W_{X^0} \cdot
W_{X^\infty} \simeq W_{X^0} \times W_{X^\infty}$ where $W_{X^0}$ is finite and
all the irreducible components of $W_{X^\infty}$" > are infinite. Let
$Y^\infty$ be the set of $t$ in $S$ such that $m_{s,t}=2$" > for all $s\in
X^\infty$. We prove that the commensurator of $W_X$ is $W_{Y^\infty} \cdot
W_{X^\infty} \simeq W_{Y^\infty} \times W_{X^\infty}$. In particular, the
commensurator of a parabolic subgroup is a parabolic subgroup, and $W_X$ is its
own commensurator if and only if $X^0=Y^\infty$.
