We investigate numbers of faces of polytopes. We begin with the face numbers of 3-polytopes. The characterisation of $f$-vectors of $d$-polytopes ($d\ge 4$) is beyond our current means.In view of this, researchers have considered characterisations of the "projections" of the $f$-vectors, namely the proper subsequences of the $f$-vector; we review the existing results. Section 8.2 gives a proof of a theorem of Xue (2021) on the minimum number of faces of $d$-polytopes with at most $2d$ vertices, answering a conjecture of Grunbaum (2003). This is followed by results on the minimum number of faces of $d$-polytopes with more than $2d$ vertices. We then discuss the lower and upper bound theorems for simplicial polytopes, due to Barnette (1973) and McMullen (1970), respectively, and their extensions such as the $g$-conjecture of McMullen (1971), now the $g$-theorem. The proof of the lower bound theorem connects rigidity theory and the combinatorics polytopes. The chapter ends with a discussion of the flag vector of a polytope. This includes a result of Bayer and Billera (1985) on linear equations for flag vectors like the Dehn--Sommerville’s equations for simplicial polytopes.

This chapter takes the concept of rigid origami and puts it in motion, studying how a crease pattern flexes from the unfolded state to a continuum of rigid origami states.The treatment presented starts with the more general theory of flexible polyhedral surfaces, then moves to the special case of origami.The configuration space of the rigid foldings of a crease pattern is introduced, and the tools of reciprocal-parallel and reciprocal diagrams are used to establish conditions for infinitesimal and second-order rigid foldability.This is used to prove that a single-vertex origami crease pattern has a rigid folding from the unfolded state if and only if it has a nontrivial zero-area reciprocal diagram.These results are then used to establish equations for the folding angles of a degree-4 flat-foldable vertex that are linear when parameterized by the tangent of half the folding angles, also known as the Weierstrass transformation.An intrinsic condition for an origami vertex crease pattern to be rigidly foldable from the unfolded state is also given.