key vs harmonize

key is a noun but harmonize is not a noun.

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

As verbs, harmonize is a hypernym of key; that is, harmonize is a word with a broader meaning than key:

Other hypernyms of key include harmonise, reconcile.

As verbs, harmonize is a hypernym of key; that is, harmonize is a word with a broader meaning than key:

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

Other hypernyms of key include chord, harmonise.

key (noun)	harmonize (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

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

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