(1)perpendicular to →a,→b,→c and →d
(2) parallel tothevectors (→a×→b)×(→c×→d)
(3) parallel to the line of intersection of the plane containing →a and →b and the plane containing →c and →d
(4) perpendicular to the line of intersection of the plane containing →a and →b and the plane containing →c and →d