Let $G$ an agebraic group and $X$ it's spherical building, that is, $X$ is the set of maximal proper parabolic subgroups of $G$ and the simplexes of $X$ are the finite set of $X$ of the form $S=\{P_{1},...,P_{r}\}$, where $P_{i}\in X$, such taht $P_{1}\cap ....\cap P_{r}$ is parapolic subgroup. My question is : what is the appartement of $X$
