metropolis vs Frankfurt on the Main

metropolis and Frankfurt on the Main both are nouns.

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

metropolis (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