Let $(W,S)$ be a finite Weyl group and let $win W$. It is widely appreciated that the descent set D(w)={sin S | l(ws)<l(w)} determines a very large and important chapter in the study of Coxeter groups. In this paper we generalize some of those results to the situation of the Bruhat poset $W^J$ where $Jsubseteq S$. Our main results here include the identification of a certain subset $S^Jsubseteq W^J$ that convincingly plays the role of $Ssubseteq W$, at least from the point of view of descent sets and related geometry. The point here is to use this resulting {em descent system} $(W^J,S^J)$ to explicitly encode some of the geometry and combinatorics that is intrinsic to the poset $W^J$. In particular, we arrive at the notion of an {em augmented poset}, and we identify the {em combinatorially smooth} subsets $Jsubseteq S$ that have special geometric significance in terms of a certain corresponding torus embedding $X(J)$. The theory of $mathscr{J}$-irreducible monoids provides an essential tool in arriving at our main results.
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