  Projection maps which appear in the theory of buildings and oriented matroids
are closely related to the notion of shellability. This was first observed by
Bj{\"o}rner. In this paper, we give an axiomatic treatment of either concept
and show their equivalence. We also axiomatize duality in this setting. As
applications of these ideas, we prove a duality theorem on buildings and give a
geometric interpretation of the flag $h$ vector. The former may be regarded as
a $q$-analogue of the Dehn-Sommerville equations. We also briefly discuss the
connection with the random walks introduced by Bidigare, Hanlon and Rockmore.
