Definition of Doubletree

Babylon English Dictionary
American network of luxury hotels specializing in business travel
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Doubletree Definition from Language, Idioms & Slang Dictionaries & Glossaries
Webster's Revised Unabridged Dictionary (1913)
(n.)
The bar, or crosspiece, of a carriage, to which the singletrees are attached.
  
Webster's Revised Unabridged Dictionary (1913), edited by Noah Porter. About
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doubletree
\dou"ble*tree`\ (?), n. the bar, or crosspiece, of a carriage, to which the singletrees are attached.

WordNet 2.0

Noun
1. a crossbar on a wagon or carriage to which two whiffletrees are attached in order to harness two horses abreast
(hypernym) crossbar