keyhole neck vs keyhole neckline

keyhole neck and keyhole neckline both are nouns.

Word	Noun	Adjective	Verb	Adverb
keyhole neck	Yes	No	No	No
keyhole neckline	Yes	No	No	No

As nouns, keyhole neck and keyhole neckline are synonyms defined as:

keyhole neck and keyhole neckline: A neckline that forms a hole bellow the collarbones.

keyhole neck (noun)	keyhole neckline (noun)
A neckline that forms a hole bellow the collarbones.	A neckline that forms a hole bellow the collarbones.