  Given a real finite hyperplane arrangement A and a point p not on any of the
hyperplanes, we define an arrangement vo(A,p), called the *valid order
arrangement*, whose regions correspond to the different orders in which a line
through p can cross the hyperplanes in A. If A is the set of affine spans of
the facets of a convex polytope P and p lies in the interior of P, then the
valid orderings with respect to p are just the line shellings of p where the
shelling line contains p. When p is sufficiently generic, the intersection
lattice of vo(A,p) is the *Dilworth truncation* of the semicone of A. Various
applications and examples are given. For instance, we determine the maximum
number of line shellings of a d-polytope with m facets when the shelling line
contains a fixed point p. If P is the order polytope of a poset, then the sets
of facets visible from a point involve a generalization of chromatic
polynomials related to list colorings.
