140. There is, however, one important exception to the rule just given. When one of the two notes forming the dissonant interval is the tonic or dominant, and the modulation is made at that point (sometimes even when no modulation is made), a leap of a dissonant interval in the subject will often become a leap of a consonance in the answer, and vice versa. We give some examples—
Here both the B and A of the subject are answered by E, on the principle explained in § 88. Our next example shows the reverse case—an octave in the subject becoming a seventh in the answer—
Here there is an implied modulation, and the change to the dominant key is assumed as early as possible (§ 121). Obviously it cannot be before the third bar at (a). The first G is treated as third of tonic, and the second as sixth of dominant (§ 128).
141. In our example (a) of § 110, we see a seventh in the subject becoming a sixth in the answer. The following interesting passage illustrates both the rule and the exception that we are now discussing—
The tonal change at the beginning of the answer alters the seventh into a sixth, but the claims of the tonal answer having been satisfied in the first bar, the augmented and diminished intervals in the second and third bars are exactly imitated.
