Entry 1 sense Webster, 1913 Doubletree /dŭb'-əl-trēˌ/ · Dou·ble·tree · IPA /dˈʌbəltrˌiː/ 01 n. The bar, or crosspiece, of a carriage, to which the singletrees are attached. 1. The bar, or crosspiece, of a carriage, to which the singletrees are attached.