  We consider the propagation of Type I open superstrings on orbifolds with
four non-compact dimensions and $N=1$ supersymmetry. In this paper, we
concentrate on a non-trivial Z_2xZ_2 example. We show that consistency
conditions, arising from tadpole cancellation and algebraic sources, require
the existence of three sets of Dirichlet 5-branes. We discuss fully the
enhancements of the spectrum when these 5-branes intersect. An amusing
attribute of these models is the importance of the tree-level (in Type I
language) superpotential to the consistent relationship between Higgsing and
the motions of 5-branes.
