swingletree

/ˈswɪŋɡəlˌtri/
noun
  1. A pivoted crossbar to which the traces of a harness are attached, allowing a horse to pull a vehicle or plow evenly.
    • Without a swingletree, the wagon would veer to one side.
    • The swingletree balanced the pull between the two horses.
    • He oiled the swingletree to prevent it from squeaking.