knob vs nailhead

knob and nailhead both are nouns.

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

As nouns, nailhead is a hyponym of knob; that is, nailhead is a word with a more specific, narrower meaning than knob:

Other hyponyms of knob include knobble.

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