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Three types of motion describe how voices move between consecutive chords: parallel (both voices move in the same direction by the same interval), contrary (voices move in opposite directions), and oblique (one voice moves while the other stays on the same note). Contrary motion is valued for creating voice independence, while parallel motion risks parallel fifths or octaves. The motion between soprano and bass is especially critical to voice leading quality.
Draw melody lines for each voice on a staff, then trace the direction and interval of movement between each pair of adjacent voices. Compare Bach chorale excerpts that use each motion type.
You know from voice-leading basics that smooth, connected motion between chords is the goal. Motion types give you a vocabulary for describing and controlling exactly how that connection happens. When you can name the motion between two voices, you can evaluate it consciously rather than just reacting to whether it "sounds right." More importantly, knowing that contrary motion between the outer voices is a structural tool — not just a stylistic preference — changes how you approach chord-to-chord writing.
Parallel motion means both voices move in the same direction by the same interval. Parallel thirds and sixths are the foundation of harmonic texture — when soprano and alto move up a third together, they reinforce the chord's color without competing. But parallel perfect intervals are a different matter: parallel fifths and octaves cause two voices to merge acoustically, losing their independence. Your ear hears one voice instead of two. This is why they're forbidden in traditional part writing — not as an arbitrary rule but because voice independence is the structural goal, and parallel perfects destroy it. Parallel thirds and sixths, by contrast, reinforce independence because the voices move together but remain distinct in pitch.
Contrary motion — voices moving in opposite directions — is the most powerful tool for voice independence. When soprano moves up and bass moves down, the texture opens; when soprano moves down and bass moves up, it closes. This breathing quality gives music a sense of physical space. More importantly, contrary motion structurally guarantees that parallel fifths and octaves cannot occur between the two voices moving in opposite directions (if they're a fifth apart going up, they can't still be a fifth apart going down). This is why contrary motion between soprano and bass is so strongly recommended: it protects voice independence and prevents the most common voice-leading errors simultaneously.
Oblique motion — one voice held while another moves — is quieter but equally important. It creates a pedal-point effect even at short range: the sustained voice provides stability while the moving voice creates motion against it. In inner voices, oblique motion often appears as a common tone between two chords, which is one of the smoothest possible connections (no motion at all in that voice). At cadences, oblique motion in the bass (holding tonic while upper voices resolve) creates a sense of arrival and settledness. The practical lesson is this: when you approach a chord change, look for held common tones first — they give you oblique motion for free. Then resolve the remaining voices by step or small leap, using contrary motion between soprano and bass wherever possible. These three motion types together, applied deliberately, are the full toolkit of smooth voice leading.
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