A swingletree or singletree 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.

What does swingletree?

swingletree. / (ˈswɪŋɡəlˌtriː) / noun. a crossbar in a horse's harness to which the ends of the traces are attachedAlso called: whippletree, (esp US) whiffletree.

Why is it called a singletree?

The name singletree came from the same medieval word of the same use. Regionally it is called a whiffletree as well, or sometimes even a horsebeam. How does a singletree work? The action of a singletree is to balance the pull on the animals alternate shoulders as he walks.