city vs Frankfurt on the Main

city and Frankfurt on the Main both are nouns.

Word	Noun	Adjective	Verb	Adverb
city	Yes	No	No	No
Frankfurt on the Main	Yes	No	No	No

city (noun)	Frankfurt on the Main (noun)
people living in a large densely populated municipality	a German city; an industrial and commercial and financial center
a large and densely populated urban area; may include several independent administrative districts
an incorporated administrative district established by state charter