  We study the behaviour of differential forms in a manifold having at least
one of their maximal isotropic local distributions endowed with the special
algebraic property of being decomposable. We show that they can be represented
as the sum of a form with constant coefficients and one that vanishes whenever
contracted with vector fields in the former distribution, provided some simple
integrability conditions are ensured. We also classify possible 'canonical
coordinates' for a certain class of forms with potential applications in
classical field theory.
