Swingletree
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A swingletree (British Isles) or singletree (North America) balances the pull of a draught horse 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 swing fore-and-aft (in a horizontal plane). The traces (the straps by which the animal pulls) attach to its ends, and the vehicle is pulled from its middle.
The centre of the swingletree may pull on chains attached to the ends of the axle, or it may be bolted directly to the body of the vehicle.
The action 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.
