  In the equivalent single layer theories of anisotropic laminates, the
coupling tensor B describes the relation between the in- and out-of plane
behavior of the plate. This tensor has some peculiar characteristics and in
particular it is not defined, so it is normally considered as an unbounded
tensor. We show in this paper a way to determine some relations between the
polar invariants of B and those of the tensors describing the in- and out-of-
plane behaviors of the laminate, A and D respectively. These relations
constitute a set of bounds for the invariants of all the tensors of the
laminate, A,B and D. It is hence shown that also the components of B must
satisfy some conditions. Some peculiar cases, interesting for applications, are
also considered.
