thumb|right|A twin-tailed [[B-25 Mitchell in flight]]
thumb|a twin-tailed [[A-10 flying]]
A twin tail is a type of vertical tail arrangement found on the empennage of some aircraft. Two vertical tails—often smaller individually than a single conventional vertical tail would be—are mounted at the outside of the aircraft's horizontal stabilizer. This arrangement is also known as an H-tail, as it resembles a capital "H" when viewed from the rear.
The twin tail was particularly used on a wide variety of World War II multi-engine designs that saw mass production, especially on the American B-24 Liberator and B-25 Mitchell bombers, the British Avro Lancaster and Handley Page Halifax heavy bombers, and the Soviet Union's Petlyakov Pe-2 attack bomber.
It can be easily confused for the similarly named twin-boom (or "double tail") arrangement, which has two separate tail-booms (typically parallel, extending aft from the wing or wing-mounted engines), rather than a single tail with twin stabilizers (a singular "twin tail" vs. two identical tails).
One variation on the twin tail is the triple tail, but the twin-boom arrangement can also be considered a variation of the twin tail.
Design
thumb|Twin tail of an [[Avro Lancaster]]
thumb|right|The twin tail of a [[Chrislea Super Ace, built in 1948]]
thumb|High-mounted twin tail of a [[Blackburn Beverley transport]]
thumb|Twin tail of a [[Textron AirLand Scorpion ISR aircraft]]
Twin tails in an H-tail configuration are outboard of the turbulence created by the wider fuselage, improving stability and control especially at high angles of attack when the wider fuselage can interfere with airflow over the vertical tail.
Separating the control surfaces allows for additional rudder area or vertical surface without requiring a massive single tail. On multi-engine, propeller designs, twin fin and rudders operating in the propeller slipstreams give greater rudder authority and improved control at low airspeeds, and one-engine-out situations, and when taxiing. H-tails may sometimes provide "end plate effect" that improves the effectiveness of the horizontal tail.
A twin tail can also simplify hangar requirements,
It also affords a degree of redundancy: if one tail is damaged, the other may remain functional. Further the dihedral angle may reduce radar-reflection by the vertical tail, reducing aircraft detectability (desirable for combat aircraft). and Grumman F-14 Tomcat.
H-tails can, in some cases, conceal hot jet exhausts from infrared-guided weapons, as in the design of the Fairchild Republic A-10 Thunderbolt II attack jet.