_Do_ is the keynote, _sol_ is the dominant.

Place on this dominant two-thirds--_si-re_--and you have the perfect dominant chord.

Add a third _fa_ and you have the famous dominant seventh, a dissonance which to-day seems actually agreeable.

Not so long ago they thought that they ought to prepare for the dissonance.

In the Sixteenth Century it was not regarded as admissible at all, for one hears the two notes _si_ and _fa_ simultaneously and this seems intolerable to the ear.