upright piano vs upright

upright piano and upright both are nouns.

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

As nouns, upright piano and upright are synonyms defined as:

upright piano (noun)	upright (noun)
a piano with a vertical sounding board	a piano with a vertical sounding board
	a vertical structural member as a post or stake

upright piano (adjective)	upright (adjective)
	upright in position or posture
	of moral excellence
	in a vertical position; not sloping