 We study the  problem of guarding an orthogonal polyhedron having reflex edges in just two directions (as opposed to three  ) by placing  guards on reflex edges only.
<meaning-changed>  We study the  problem of guarding an orthogonal polyhedron having reflex edges in just two directions (as opposed to three  ) by placing  guards on reflex edges only.
Let an orthogonal polyhedron be the union of a finite set of boxes in \mathbb R^3 (i.e., cuboids with edges parallel to the coordinate axes), whose surface is a connected 2-manifold. We study the problem of guarding an orthogonal polyhedron having reflex edges in just two directions (as opposed to three ) by placing guards on reflex edges only.