DUODECIMAL is a 10 letter word that starts with D.

1. \Du`o*dec"i*mal\, a. [L. duodecim twelve. See
   {Dozen}.]
   Proceeding in computation by twelves; expressed in the scale
   of twelves. -- {Du`o*dec"i*mal*ly}, adv.
   
   

2. \Du`o*dec"i*mal\, n.
   1. A twelfth part; as, the duodecimals of an inch.
   
   2. pl. (Arch.) A system of numbers, whose denominations rise
      in a scale of twelves, as of feet and inches. The system
      is used chiefly by artificers in computing the superficial
      and solid contents of their work.