home key vs tonic key

home key and tonic key both are nouns.

Word	Noun	Adjective	Verb	Adverb
home key	Yes	No	No	No
tonic key	Yes	No	No	No

As nouns, home key and tonic key are synonyms defined as:

home key and tonic key: the basic key in which a piece of music is written

home key (noun)	tonic key (noun)
the basic key in which a piece of music is written	the basic key in which a piece of music is written