nailhead vs knob

nailhead and knob both are nouns.

Word	Noun	Adjective	Verb	Adverb
nailhead	Yes	No	No	No
knob	Yes	No	No	No

As nouns, knob is a hypernym of nailhead; that is, knob is a word with a broader meaning than nailhead:

Other hypernyms of nailhead include boss.

nailhead (noun)	knob (noun)
flattened boss on the end of nail opposite to the point	a round handle
something resembling the head of a nail that is used as an ornamental device	a circular rounded projection or protuberance
	an ornament in the shape of a ball on the hilt of a sword or dagger
	any thickened enlargement