• WordNet 3.6
    • n swingletree a crossbar that is attached to the traces of a draft horse and to the vehicle or implement that the horse is pulling
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Webster's Revised Unabridged Dictionary
    • n Swingletree A whiffletree, or whippletree. See Singletree.
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Century Dictionary and Cyclopedia
    • n swingletree A cross-bar, pivoted at the middle, to which the traces are fastened in a cart, carriage, plow, etc. From singletree, a corruption of swingletree, arose the name doubletree for the equalizing-bar to which a pair of animals is hitched by means of a pair of swingletrees, each center-bolted and swinging freely like the doubletree itself. The extent of swing of the doubletree is generally limited by a chain or strap passing to the fore axle on each side. The swingletree gives freedom of alternating action to the shoulders of the horse, and also prevents that motion from being communicated to the vehicle. In the case of the doubletree it further correlates and equalizes the traction of the two animals composing the team. Also swingtree, whiffletree.
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Webster's Revised Unabridged Dictionary
So named in allusion to its swinging,. See Swingle (v. i.), and cf. Swingtree


In literature:

The swingletree gear next to it probably was left there during the construction.
"The Cultural History of Marlborough, Virginia" by C. Malcolm Watkins