 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.
 We study the problem of guarding an orthogonal polyhedron having reflex edges in just two directions (as opposed to three ) by placing the minimum number of edge guards on reflex edges only.