keynote (n.)

also key-note, "lowest note of a musical scale, basis of a tonal key, the tonic," 1776, from key (n.1) in sense of "musical scale" + note (n.). The keynote is the given note on which the melodic and harmonic relationships in the scale are built, and it gives its name to the key. Figurative sense of "leading idea, central principle" is from 1783; keynote speech is 1887, American English.