A crossbar that is attached to the traces of a draft horse and to the vehicle or implement that the horse is pulling

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Swingletree \Swin"gle*tree`\, n. [So named in allusion to its swinging. See Swingle , v. i., and cf. Swingtree .] A whiffletree, or whippletree. See Singletree .

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
mid-15c., from swingle "that which swings," from swing (v.) + instrumental suffix -le (compare handle ), + tree (n.) in obsolete Middle English sense "pole."

Wiktionary Word definitions in Wiktionary
n. a whippletree

WordNet Word definitions in WordNet
n. a crossbar that is attached to the traces of a draft horse and to the vehicle or implement that the horse is pulling [syn: whiffletree , whippletree ]

Wikipedia Word definitions in Wikipedia
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 ...