Note 27 — Chapter IV · THE PRINCIPLES OF PURE MUSIC

The reason of the “answer at the fifth” is this: the tonic and dominant being the two tonal centres of the key, about which all its sounds are grouped, it is natural that they should be treated as complementary to each other and made the bases of contrast effects. After the subject is announced in the tonic, then, it is answered in the dominant, or a fifth above (or a fourth below, which amounts to the same thing). See Figure VIII.