The Collaborative International Dictionary of English v.0.48:

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

The Collaborative International Dictionary of English v.0.48:

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]