grandfather clock vs longcase clock

grandfather clock and longcase clock both are nouns.

Word	Noun	Adjective	Verb	Adverb
grandfather clock	Yes	No	No	No
longcase clock	Yes	No	No	No

As nouns, grandfather clock and longcase clock are synonyms defined as:

grandfather clock and longcase clock: a pendulum clock enclosed in a tall narrow case

grandfather clock (noun)	longcase clock (noun)
a pendulum clock enclosed in a tall narrow case	a pendulum clock enclosed in a tall narrow case