1) Crossbar 2) Whiffletree 3) Whippletree 1) Swiveltree 2) Whiffletree 3) Whippletree 
A swingletree (British Isles) or singletree (North America) is a wooden or metal bar used to balance the pull of a draught horse or other draught animal when pulling a vehicle. It is a kind of whippletree, and the term is also used sometimes for other whippletrees. It is a horizontal bar, attached or suspended in the middle, and able to hinge fore...
• (v. i.) A whiffletree, or whippletree. See Singletree.
Swin'gle·tree` noun [ So named in allusion to its swinging . See Swingle , intransitive verb , and confer Swingtree .] A whiffletree, or whippletree. See Singletree . 
crossbar pivoted in the middle, to which the traces are attached in a horse-drawn cart or plough.
a whiffletree.