knob vs doorknob

knob and doorknob both are nouns.

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

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

knob: a round handle
doorknob: a knob used to release the catch when opening a door (often called `doorhandle' in Great Britain)

Other hyponyms of knob include doorhandle, stop.

knob (noun)	doorknob (noun)
a round handle	a knob used to release the catch when opening a door (often called `doorhandle' in Great Britain)
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