Source : Webster's Revised Unabridged Dictionary (1913)

Singletree \Sin"gle*tree`\, n. [Cf. {Swingletree}.]
   The pivoted or swinging bar to which the traces of a
   harnessed horse are fixed; a whiffletree.

   Note: When two horses draw abreast, a singletree is fixed at
         each end of another crosspiece, called the doubletree.