key tone

Webster's Dictionary (GCIDE)

Key tone \Key" tone`\n. (Mus.) See Keynote. [1913 Webster]

Keynote \Key"note`\, n.

1.(Mus.) The tonic or first tone of the scale in which a piece or passage is written; the fundamental tone of the chord, to which all the modulations of the piece are referred; -- called also key tone. [1913 Webster]

2.The fundamental fact or idea; that which gives the key; as, the keynote of a policy or a sermon. [1913 Webster]