stool vs piano stool

stool and piano stool both are nouns.

Word	Noun	Adjective	Verb	Adverb
stool	Yes	No	Yes	No
piano stool	Yes	No	No	No

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

stool (noun)	piano stool (noun)
a simple seat without a back or arms	a stool for piano players; usually adjustable in height
a plumbing fixture for defecation and urination
(forestry) the stump of a tree that has been felled or headed for the production of saplings
solid excretory product evacuated from the bowels

stool (verb)	piano stool (verb)
have a bowel movement
grow shoots in the form of stools or tillers
react to a decoy, of wildfowl
lure with a stool, as of wild fowl