Lagrangian subbundles and codimension 3 subcanonical subschemes

We show that a Gorenstein subcanonical codimension 3 subscheme Z subset of X = P-N, N greater than or equal to 4, can be realized as the locus along w hich two Lagrangian subbundles of a twisted orthogonal bundle meet degenera tely and conversely. We extend this result to singular Z and all quasi-proj ective ambient schemes X under the necessary hypothesis that Z is strongly subcanonical in a sense defined below. A central point is that a pair of La grangian subbundles can be transformed locally into an alternating map. In the local case our structure theorem reduces to that of D. Buchsbaum and D. Eisenbud [6] and says that Z is Pfaffian. We also prove codimension 1 symmetric and skew-symmetric analogues of our s tructure theorems.