neckline vs crew neck

neckline and crew neck both are nouns.

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

As nouns, crew neck is a hyponym of neckline; that is, crew neck is a word with a more specific, narrower meaning than neckline:

neckline: the line formed by the edge of a garment around the neck
crew neck: a plain straight neckline opening from shoulder to shoulder of sweaters

Other hyponyms of neckline include crew neckline, decolletage, keyhole neck, keyhole neckline.

neckline (noun)	crew neck (noun)
the line formed by the edge of a garment around the neck	a plain straight neckline opening from shoulder to shoulder of sweaters