Consider the diagram. planes m and n intersect at line d. lines a, b, and e are on plane m. line a is vertical and forms a right angle with line d. line b is diagonal and goes up and to the right. line e is at the top of the plane and is close to being horizontal. line c is on plane n and goes slightly up and to the right. lines a and d are non-coplanar. parallel. perpendicular. skew.