subtitled, What I forgot to teach you about symmetrical divisions of the octave…
The omnibus sequence is a special type of harmonic sequence. When the sequential unit moves by minor third a chromatic scale will occur in one voice, usually the bass. The omnibus sequence is different than a regular harmonic sequence, in that the individual voices do not all move in parallel (strict or diatonic) motion between units.
The features of the omnibus sequence are:
- The sequential unit is a three-chord pattern.
- The unit pattern pivots around a 64 triad (a second inversion triad will always be the second chord of the unit pattern).
- Two voices will move in contrary chromatic motion (the bass and one upper voice).
- Two voices will not move within the unit pattern.
- The upper voice chromatic motion will always switch between voices from unit to unit. (It will cycle through the upper voices.)
- There are two types of omnibus sequences. One starts with a °7 chord, with the chromatic scale ascending. The other starts with a Ger+65 chord, with the chromatic scale descending.
- Enharmonic spellings are frequently used.
The °7 omnibus
- The sequential unit is °7 – 64 – Ger+65 (or °3).
- The contrary motion is not a perfect mirror. The bass moves by half step throughout, but the upper voice will first descend a M2 (or °3), then a m2. Look for the upper voice that can descend that distance to double the bass of the second chord.
- With root movement of a minor third (between units), the beginning °7 chord of each unit will be made up of the same pitches (probably respelled enharmonically).
- All the upper voices will remain common from the third chord of one sequential unit to the first chord of the next sequential unit. The bass will continue its chromatic ascent.
The Ger+6 omnibus
- The harmonic pattern of the unit is Ger65 – 64 – Ger°3.
- The contrary chromatic motion is strictly symmetrical. Both voices move m2 – m2.
- Two voices still remain as common tones throughout the unit pattern.
- Each unit contains a voice exchange between the two voices that start with the +6, ending with a °3. For example, if the +6 is Ab to F#, the Ab will descend to G then F# (the starting point of the other voice), while the F# will ascend to G then Ab (the starting point of the other voice).
- The contrary chromatic voices will continue their outward m2 progression from the third chord of one unit to the first chord of the next unit.
- The stationary voices in one unit will remain common from chord three of one unit to chord one of the next. (After this, one of the two common tone voices will form an +6 with the bass and will become the contrary motion voice.)
Enharmonic Spelling Choices
My in-class examples tried to maintain the +6 intervals wherever possible for maximum clarity of explanation. You do not need to be so slavish to proper interval spelling. My only guideline is that octaves should be spelled as such (C – C), not as augmented sevenths (C – B#) or diminished ninths (C – Dbb).
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