  It is well known that every smooth cubic threefold is the zero locus of the
Pfaffian of a 6 x 6 skew-symmetric matrix of linear forms in P^4. To compactify
the space of such Pfaffian representations of a given cubic and to study the
construction in families as well as for singular or reducible cubics, it is
thus natural to consider the incidence correspondence of Pfaffian
representations inside the product of the space of semistable skew-symmetric 6
x 6 matrices of linear forms in P^4 and the space of cubics. Here we describe
concretely the irreducible component of this incidence correspondence
dominating the space of skew matrices.
