A little trivia on the question
Sometimes Duchies and Principalities are called Dukedoms and Princedoms respectively. I know that is
not modern parlance, but 500+ years ago the terminology was a little less exact
Two others
Archduke - Archduchy
Great-Count - Great-County
The latter is slightly obscure, I only know of one state that maintained the title in all of history (Sicily before it became a Kingdom). The former was mostly restricted the Germany I believe.
Oh,
Elector - Electorate
Another of those wonderful HRE situations.
Incidentally, sometimes you will hear a marquis being associated with a 'marquisite'. (sp?)
Also their is a complication that until *relatively* recently the word 'prince' meant, in addition to the other meaning, simply a monarchical ruler (because of the Roman precedent). Hence 'The Prince' by Machiaevelli and the fact that a number of monarchs might refer to themselves as being 'Princes' - like Elizabeth I (and she called herself a Prince, NOT a Princess).