pos·til·ion /poˈstɪljən, pə-/
  左馬御者;平頂女帽
  Pos·til·ion n.  One who rides and guides the first pair of horses of a coach or post chaise; also, one who rides one of the horses when one pair only is used. [Written also postillion.]
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  postilion
       n : someone who rides the near horse of a pair in order to guide
           the horses pulling a carriage (especially a carriage
           without a coachman) [syn: postillion]