(1) The keynote of any scale. The ground tone or basis of a scale or key. (2) The key-chord in which a piece is written, and with which it concludes. [Stainer, John; Barrett, W.A.; A Dictionary of Musical Terms; Novello, Ewer and Co., London, pre-1900]
The subdominant, or lowest chord in the key - F, A, C, is the natural product of the first combination of the three primary ratios (2,3 ,5). Their second combination develops the tonic or middle chord C, E, G. The third combination develops the dominant or highest chord G, B, D.
A letter system of notation. Many attempts have from time to time been made to produce a simpler notation than the stave, clefs, signature, etc. of the so-called "Old Notation". [Stainer, John; Barrett, W.A.; A Dictionary of Musical Terms; Novello, Ewer and Co., London, pre-1900]
"By affinity the notes group in chords. The tonic is the center chord, the key of harmony; the dominant is the fifth above it and the subdominant the fifth below it, and these two are balanced on the center chord as the scales on a balance beam. The dominant chord is vigorous and active, tending to soar; the subdominant is solemn, soft, and grave, tending to sink; the tonic is melodious and restful, and in it the harmony of equilibrium. This far AFFINITY." [Scientific Basis and Build of Music, page 91]