longcase clock vs grandfather clock

longcase clock and grandfather clock both are nouns.

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

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

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

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