What's the difference between doubletree and singletree?

Doubletree

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Definition:

• (n.) The bar, or crosspiece, of a carriage, to which the singletrees are attached.

Example Sentences:

• (1) She and Gaiman stay at the Ritz-Carlton , while I stay at the crappier DoubleTree down the road.

Singletree

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Definition:

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

Example Sentences: