['dʌbltri:]
n.
(用双马拉车等时使用的)双驾横木

[ noun ]
a crossbar on a wagon or carriage to which two whiffletrees are attached in order to harness two horses abreast
<noun.artifact>

Doubletree \Dou"ble*tree`\, n.
The bar, or crosspiece, of a carriage, to which the
singletrees are attached.