  Given a set of n unit squares in the plane, the goal is to rank them in space
in such a way that only few squares see each other vertically. We prove that
ranking the squares according to the lexicographic order of their centers
results in at most 3n-7 pairwise visibilities for n at least 4. We also show
that this bound is best possible, by exhibiting a set of n squares with at
least 3n-7 pairwise visibilities under any ranking.
