If the lower voice ascends to E, the tritone resolves from a d5 (diminished fifth) to a M3 (major third). Lower voice (with upper voice moving to G #):Stepping up to E is the only valid answer. Sustaining D # would also not be a valid answer as oblique motion is not permitted when resolving a tritone.) This is also a dissonance and cannot be used as a resolution from a tritone. If the lower voice descends to C #, the resulting interval will be a m7 (minor seventh). ( Note: Stepping down to C # would not be a valid answer. Furthermore, D # is the leading tone and must resolve to the tonic. The tritone then resolves from a d5 to a P5 (perfect fifth). If the upper voice ascends out of a d5 (diminished fifth), the lower voice must also ascend. Lower voice (with upper voice moving to B): Stepping up to E is the only valid answer.
( Note: Sustaining A would not be a valid answer as oblique motion is not permitted when resolving a tritone.) Upper voice: The upper voice may move by step to either B or G #. It is also worth mentioning that the same interval progression in reverse-movement from a P5 to a d5-is perfectly acceptable and happens regularly in common practice. With permissible interval progressions occurring between the bass and each of the upper voices, the tritone in each case may be thought of as a resultant interval, and the similar-motion progression as a byproduct. The interval progressions formed with the bass validate the similar motion in the upper voices. Now consider the intervals formed with the bass the alto moves in parallel sixths with the bass while the soprano moves in parallel thirds with the bass. The alto and soprano voices form a tritone on beat three which resolves to a perfect fifth on beat four. In both cases, one of the voices moves in parallel thirds with the bass while the other creates parallel sixths with the bass. Their validity is explained by the interval progressions formed with the bass. Similar-motion resolutions of the tritone (A4 to P4 and d5 to P5) may seem counterintuitive. The third of the chord-scale degree \hat2-appears in the bass and the inversion is indicated with the bass figure 6 appearing next to the Roman numeral, as shown here: In order to avoid accentuating the dissonance in this manner, they invert the vii o chord. When one of the tritone forming notes-the root or fifth-appears in the bass, composers generally feel that the dissonant interval sounds too harsh. We must therefore make an exception to the constraints described earlier when we use this chord in three- and four-part textures. The diminished fifth of the vii o chord, however, is inherently dissonant. When dissonances appeared, they were shown to be a byproduct of consonant progressions between other pairs of voices happening at the same time. In our discussions of basic interval progressions ( Chapter 12 and Chapter 14), we adhered to the practice of using only consonant intervals.
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