piano stool vs stool

piano stool and stool both are nouns.

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

As nouns, stool is a hypernym of piano stool; that is, stool is a word with a broader meaning than piano stool:

piano stool (noun)	stool (noun)
a stool for piano players; usually adjustable in height	a simple seat without a back or arms
	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

piano stool (verb)	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