Principles of Motion

Introduction: Navigating the Intersection

Chapter 7 set up the four singers' starting positions. This chapter is about how they move. When one chord changes to the next, each voice travels to a new note (or stays put), and the relationship between any two moving voices falls into one of four types.

Click any example to hear it. Red marks the problem notes.

The Four Types of Motion

1. Contrary motion: the voices move in opposite directions. This is the most independent kind of motion; use it often, especially between soprano and bass.
2. Oblique motion: one voice holds a note (a common tone) while the other moves.
3. Similar motion: both voices move in the same direction, by different intervals.
4. Parallel motion: both voices move in the same direction by the same interval, so the distance between them stays the same.

The Strict Prohibitions

Four independent voices only sound independent if no two of them fuse together. Perfect consonances are so stable and "hollow" that two voices moving in parallel perfect intervals blend into one thicker line. So classical four-part style forbids:

• Parallel fifths: two voices a perfect fifth (or twelfth) apart that both move to another perfect fifth.
• Parallel octaves and unisons: two voices an octave (or unison) apart that move to another octave or unison. They stop being two parts: one simply doubles the other.
• Contrary fifths and octaves: the same perfect interval reached by contrary motion (a fifth becoming a twelfth, an octave becoming a unison or fifteenth). It's the same fusion in disguise.

Parallel imperfect consonances (thirds and sixths) are fine, and in fact beautiful: they're the sound of so much choral and folk music.

These rules aren't arbitrary. They're acoustic guardrails that protect each line's independence. When you write for orchestra, you'll double melodies in octaves all the time to add weight. But in strict four-part SATB writing, they're out.

Direct (Hidden) Fifths and Octaves

A subtler problem: when the two outer voices (soprano and bass) move in similar motion into a perfect fifth or octave, the ear still hears a fused, hollow arrival, especially if the soprano leaps. These are called direct (or hidden) fifths and octaves.

• Avoid them between soprano and bass when the soprano leaps.
• They're acceptable when the soprano moves by step (common at cadences, for example V–I with the soprano 2̂–1̂).
• Between inner voices, or between an inner voice and the bass, they're generally fine.

Unequal Fifths

A diminished fifth moving to a perfect fifth (d5 → P5) in parallel motion is called an unequal fifth. Avoid it when it involves the bass. The reverse, P5 → d5, is fine, and so is d5 → P5 between two upper voices when the d5 resolves normally.

Resolving Tendency Tones

The active scale degrees from Chapter 3 now become voice-leading obligations:

• The leading tone (7̂) rises to the tonic (1̂) when V (or vii°) moves to I. In the soprano or bass this is required. In an inner voice, it may instead drop to 5̂ so the tonic chord can be complete, a common exception in V–I.
• The chordal seventh falls by step. In V⁷ the seventh is 4̂, and it resolves down to 3̂. The same holds for every seventh chord (ii⁷, vii^ø7, and so on).
• Together, these two resolutions resolve the tritone in V⁷: 7̂ and 4̂ move inward (or outward, as an inverted tritone) to 1̂ and 3̂.

Connecting Two Chords: A Strategy

Most voice-leading problems disappear if you connect chords in this order:

1. Write the bass (it's often given).
2. Keep common tones in the same voice. I and IV share 1̂; I and V share 5̂.
3. Move every other upper voice to the nearest available chord tone, usually by step.
4. When chords share no common tone (IV–V, V–vi), move the three upper voices in contrary motion to the bass. This one habit prevents most parallel fifths and octaves.
5. Resolve tendency tones, then check doubling and spacing (Chapter 7).
6. Audit every pair of voices (all six pairs) for parallel and contrary fifths and octaves.

Try It: Find the Parallels

The progression in the checker has several voice-leading errors. Find them, then fix them one by one until the checker comes up clean.

Check Your Understanding