1. 
   Single tree
   Single whiffletree
   Found op http://www.gaitedhorses.net/Articles/HorseGlossary.html
   
   
2. 
   Singletree
   Sin'gle·tree` noun [ Confer Swingletree .] The pivoted or swinging bar to which the traces of a harnessed horse are fixed; a whiffletree. » When two horses draw abreast, a singletree is fixed at each end of another crosspiece, called ...
   Found op http://www.encyclo.co.uk/webster/S/107
   
   
3. 
   Singletree
   • (n.) The pivoted or swinging bar to which the traces of a harnessed horse are fixed; a whiffletree.
   Found op http://thinkexist.com/dictionary/meaning/singletree/
   
   

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