1. Doubletree - a Crossbar on a wagon or carriage to which two whiffletrees are attached in order to harness two horses abreast swingletree , whiffletree , whippletree - a Crossbar that is attached to the traces of a draft horse and to the vehicle or implement that the horse is pulling

2. Clevis (noun) A piece of metal bent in the form of an oxbow, with the two ends perforated to receive a pin, used on the end of the tongue of a plow, wagen, etc., to attach it to a draft chain, whiffletree, etc.; - called also clavel, clevy.