harmonize vs key

harmonize is not a noun while key is a noun.

Word	Noun	Adjective	Verb	Adverb
harmonize	No	No	Yes	No
key	Yes	Yes	Yes	No

As verbs, key is a hyponym of harmonize; that is, key is a word with a more specific, narrower meaning than harmonize:

harmonize: bring into consonance, harmony, or accord while making music or singing
key: regulate the musical pitch of

As verbs, key is a hyponym of harmonize; that is, key is a word with a more specific, narrower meaning than harmonize:

harmonize (noun)	key (noun)
	metal device shaped in such a way that when it is inserted into the appropriate lock the lock's mechanism can be rotated
	a lever (as in a keyboard) that actuates a mechanism when depressed
	mechanical device used to wind another device that is driven by a spring (as a clock)
	the central building block at the top of an arch or vault
	pitch of the voice
	something crucial for explaining
	any of 24 major or minor diatonic scales that provide the tonal framework for a piece of music
	a generic term for any device whose possession entitles the holder to a means of access
	a list of words or phrases that explain symbols or abbreviations
	a list of answers to a test
	(basketball) a space (including the foul line) in front of the basket at each end of a basketball court; usually painted a different color from the rest of the court
	a coral reef off the southern coast of Florida
	a winged often one-seed indehiscent fruit as of the ash or elm or maple
	a kilogram of a narcotic drug

harmonize (adjective)	key (adjective)
	serving as an essential component

harmonize (verb)	key (verb)
bring into consonance, harmony, or accord while making music or singing	harmonize with or adjust to
bring into consonance or accord	regulate the musical pitch of
bring (several things) into consonance or relate harmoniously	identify as in botany or biology, for example
sing or play in harmony	vandalize a car by scratching the sides with a key
write a harmony for	provide with a key
go together