  We study the problem to provide a triangular form based on implicit
differential equations for non-linear multi-input systems with respect to the
flatness property. Furthermore, we suggest a constructive method for the
transformation of a given system into that special triangular shape, if
possible. The well known Brunovsky form, which is applicable with regard to the
exact linearization problem, can be seen as special case of this implicit
triangular form. A key tool in our investigation will be the construction of
Cauchy characteristic vector fields that additionally annihilate certain
codistributions. In adapted coordinates this construction allows to single out
variables whose time-evolution can be derived without any integration.
