Hyperbolicity is shown to be
not an absolute invariant in the sense defined by the author. Specifically, an example
of a nonhyperbolic system is given with a partial prolongation which is hyperbolic. A
large class of systems is found which is closed under modified absolute equivalence
and which contains all hyperbolic systems. These ideas are applied to give existence
theorems for the initial value problem in several types of nonhyperbolic
systems.