  Let V be a rank N vector bundle on a d-dimensional complex projective scheme
X; assume that V is equipped with a quadratic form with values in a line bundle
L and that S^2 V^* \otimes L is ample. Suppose that the maximum rank of the
quadratic form at any point of X is r > 0. The main result of this paper is
that if d > N-r, then the locus of points where the rank of the quadratic form
is at most r-1 is nonempty. We give some applications to subschemes of
matrices, and to degeneracy loci associated to embeddings in projective space.
The paper concludes with an appendix on Gysin maps. The main result of the
appendix identifies a Gysin map with the natural map from ordinary to relative
cohomology.
