convex shape vs knob

convex shape and knob both are nouns.

Word	Noun	Adjective	Verb	Adverb
convex shape	Yes	No	No	No
knob	Yes	No	No	No

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

Other hyponyms of convex shape include umbo, camber, entasis, node, thickening, ridge, peak, point, tip, taper, projection.

convex shape (noun)	knob (noun)
a shape that curves or bulges outward	a round handle
	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