sin·gle·tree 
(sĭnggəl-trē′)
(sĭnggəl-trē′)n.
The pivoted horizontal crossbar to which the harness traces of a draft animal are attached and which is in turn attached to a vehicle or an implement. Also called regionally whiffletree, whippletree.
[Alteration (influenced by DOUBLETREE) of SWINGLETREE.]