Suppose M and N are PL
manifolds and f : M → N is a proper PL map. Triangulate M and N so that f is
simplical and let X be the dual complex in N. Then for each open simplex σ
in X, f−1(σ) is a PL submanifold of M, so the stratification of N by the
open simplices of X pulls back to a stratification of M. In other words, any
such PL map can be regarded as a map of combinatorially stratified sets in
which each n-stratum of therange is a disjoint union of copies of Rn. Here we
prove the analogous theorem for a smooth map f : M → N between smooth
manifolds.