  A set of nonlocal correlations that have come to be known as a PR box suggest
themselves as a natural unit of nonlocality, much as a singlet is a natural
unit of entanglement. We present two results relevant to this idea. One is that
a wide class of multipartite correlations can be simulated using local
operations on PR boxes only. We show this with an explicit scheme, which has
the interesting feature that the number of PR boxes required is related to the
computational resources necessary to represent a function defining the
multipartite box. The second result is that there are quantum multipartite
correlations, arising from measurements on a cluster state, that cannot be
simulated with n PR boxes, for any n.
