The bar, or crosspiece, of a carriage, to which the singletrees are attached.

A crosspiece with a singletree attached to each end, used when two animals are harnessed at once.

a crossbar on a wagon or carriage to which two whiffletrees are attached in order to harness two horses abreast

Doubletree is an upscale hotel chain operated by Hilton Hotels Corporation. It currently owns over 150 properties, servicing more than 40,900 rooms throughout the United States, Canada, and Latin America.