Swingletree

A swingletree (British Isles) or singletree (North America) is a small wooden or metal bar behind a harnessed horse.

The action of a swingletree is to balance the pull from alternate shoulders as the animal walks.

It is used especially when the animal is in a breastcollar harness, because this can easily rub the shoulders if the pull is uneven.

It is needed less for an animal in a horse collar, as the pull does not pass over the shoulders in the same way.

For this reason heavier vehicles may have no swingletree, as they are normally pulled with a horse collar.

A typical wooden swingletree with iron fittings
The red horizontal bar behind the horse is a swingletree
A swingletree built into a cart. The swingletree is the triangular piece in the foreground; the traces are the leather straps leading forward from this. (Swingletrees are more commonly a simple bar, not a triangle.)