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Webster 1913 Edition

Noun.

[Cf.

The pivoted or swinging bar to which the traces of a harnessed horse are fixed; a whiffletree.

☞ When two horses draw abreast, a singletree is fixed at each end of another crosspiece, called the doubletree.

singletree ‎(plural singletrees)

A bar behind draft animals and in front of a load, such as a wagon, that balances the load. Generally the animals are attached at the ends and the wagon or other load to a pivot in the middle of the singletree.