Source : Webster's Revised Unabridged Dictionary (1913)

Ducal \Du"cal\, a. [F. ducal. See {Duke}.]
   Of or pertaining to a duke.

         His ducal cap was to be exchanged for a kingly crown.
                                                  --Motley.

Source : WordNet®

ducal
     adj : of or belonging to or suitable for a duke; "ducal palace"