Closed wing: two wing planes are merged or joined structurally at or near the tips in some way.[4] This stiffens the structure and can reduce aerodynamic losses at the tips. Variants include: Box wing: upper and lower planes are joined by a vertical fin between their tips. The first officially witnessed unaided takeoff and flight, Santos-Dumont's 14-bis, used this configuration and some Dunne biplanes were of this type as well.