Every ordered collection of sets in Euclidean space can be associated to a
combinatorial code, which records the regions cut out by the sets in space. Given
two ordered collections of sets, one can form a third collection in which the
-th set
is the Cartesian product of the corresponding sets from the original collections. We
prove a general “product theorem” which characterizes the code associated to the
collection resulting from this operation, in terms of the codes associated to the
original collections. We use this theorem to characterize the codes realizable by
axis-parallel boxes, and exhibit differences between this class of codes and
those realizable by convex open or closed sets. We also use our theorem to
prove that a “monotonicity of open convexity” result of Cruz, Giusti, Itskov,
and Kronholm holds for closed sets when some assumptions are slightly
weakened.