  The cusps of the caustics of any gravitational lens model can be classified
into positive and negative ones. This distinction lies on the parity of the
images involved in the creation/destruction of pairs occurring when a source
crosses a caustic in a cusp. In this paper, we generalize the former definition
of the sign of the cusps. Then we apply it to the binary lens. We demonstrate
that the cusps on the axis joining the two lenses are positive while the others
are negative. To achieve our objective, we combine catastrophe theory, usually
employed in the derivation of the properties of caustics, with perturbative
methods, in order to simplify calculations and get readable results. Extensions
to multiple lenses are also considered.
