grand duchy vs demesne

grand duchy and demesne both are nouns.

Word	Noun	Adjective	Verb	Adverb
grand duchy	Yes	No	No	No
demesne	Yes	No	No	No

As nouns, demesne is a hypernym of grand duchy; that is, demesne is a word with a broader meaning than grand duchy:

Other hypernyms of grand duchy include domain, land.

grand duchy (noun)	demesne (noun)
the domain controlled by a grand duke or grand duchess	territory over which rule or control is exercised
	extensive landed property (especially in the country) retained by the owner for his own use