Voice leading and how parts move

The chord names say which notes sound together. Voice leading is how each part gets from its note in one chord to its note in the next. A part, or voice, is one line of notes: a singer, an instrument, or one line inside a pianist’s chords. Two pieces can use the same chords and sound very different, smooth or clumsy, because their parts move differently.

Harmony courses teach it with four-part writing: soprano, alto, tenor and bass, as a choir sings a hymn or as Bach wrote his chorales. A few rules keep each of the four a line of its own. The piano roll can check them for you, and show how each part moves, as a teacher would mark an exercise.

Checking an exercise

Tick Voice leading next to Ghost notes at the top of the piano roll. The piano roll then shows:

• A Voices row under the chord names, with a block over each chord where something classical part writing avoids happens: 5ths and 8ves for parallel fifths and octaves, 1sns for parallel unisons, LT for a leading tone that doesn’t rise, and 7th for a seventh that doesn’t fall.
• A line of colour for each part, from each of its notes to the next, so you can follow one part through the pattern as on a score. The moves into the chord you point at are drawn bolder.
• A dashed outline where a leading tone or a seventh should have gone.
• A line over the chord names that sums up the check, as in “Voice leading, 4 parts: parallel fifths (1), seventh not falling (1).” Pointing at a chord with a finding says what happens there instead.

Point at a block in the Voices row to read why part writing avoids it:

To write this exercise yourself, add an empty pattern, open the piano roll, and draw these notes a beat each, the bass first:

It has two slips, on purpose. From C to F, the tenor and the bass are a fifth apart and both rise a fourth into another fifth. From G7 to C, the soprano’s F, the seventh of G7, rises to G instead of falling to E.

Here is the same progression written well:

The line now reads “Voice leading, 4 parts: nothing classical part writing avoids.” Look at what changed: wherever two chords share a note, a part keeps it (C5 from C to F, F4 from F to G7, G4 from G7 to C), and the other parts move by step. The seventh, F4, falls to E4, and the leading tone, B3, rises to C4. That is most of the craft.

Which notes make a part

A track that plays one note at a time is one part. A track that plays chords holds several: the top note of each chord goes to the top part, the next one to the next part, and so on. The messages call them by place, “voice 3 of 4” or “lowest voice”, as in the picture, where all four parts are on one track.

With Ghost notes on, the other tracks are checked too, each by its name, so the bass can be on a track of its own. Drum tracks are left out. A rest of a beat or more ends a line. Two parts that play in octaves all the time, as basses doubling the cellos, are one line made louder, and are counted once.

The rules

No parallel fifths or octaves

Two parts a perfect fifth apart that both move the same way into another perfect fifth make parallel fifths. The same goes for octaves and unisons. These intervals blend so well that the two parts sound like one for that moment, and the music loses a line. A fifth plus an octave still counts as a fifth.

Moving the opposite way from a fifth to a twelfth makes fifths by contrary motion. Each part moves differently, but the ear still hears two fifths in a row, so strict part writing avoids those too. A held or repeated note doesn’t count: both parts have to move.

The leading tone rises

The leading tone is the note a semitone below the tonic: B in C major, G♯ in A minor. When a dominant chord (V, V7, vii°) goes to the tonic, the leading tone rises to the tonic. The rule holds in the top part and the bass, where the ear follows the line. An inner part may drop to the fifth of the tonic chord instead, so that the chord gets all its notes, and the check leaves inner parts alone.

The check also expects the leading tone to rise in a deceptive cadence, V to vi, and in a secondary dominant: in D7 to G, in C major, the F♯ rises to G (see Chords from outside the key). Classical part writing never doubles the leading tone, and the message says so when two parts have it.

A seventh falls

In a seventh chord such as G7 (G B D F), the seventh, F, is a dissonance, and it falls a step into the next chord: F to E in G7 to C. The textbook exceptions are not marked:

• the next chord has the same note, and the seventh holds it;
• a suspension: the seventh is held into the next chord and falls a little later;
• a short passing or neighbour note that comes and goes by step;
• V⁴₃ to I⁶, where the seventh may rise while the bass rises a tenth below it.

What it doesn’t check

Hidden fifths and octaves (two parts moving the same way into a fifth or an octave), the spacing between parts, parts crossing, and which note of a chord is doubled are left to you. And these are the rules of one style, the hymns and chorales of the 1700s and the harmony exercises built on them, not of all music: jazz, pop and Debussy move in parallel fifths on purpose.

The band keeps the first rule where it holds long chords, as a choir sings them: in styles such as Hymn, Romanesca or Ambient, each chord takes, of the voicings nearest the last one, the first that moves no two parts, the bass among them, in parallel fifths or octaves or in fifths or octaves by contrary motion, when there is one. From C E G over C, D minor comes as A D F, the upper parts going down while the bass goes up. Styles that play their chords in rhythm take the nearest voicing, as a pianist’s hands do.

Where the key is a twelve-tone row, the check uses the rules of twelve-tone writing instead, and a Row line shows the forms of the row it reads in the notes: see Beyond keys: sets, rows and micromodes.

How each part moves

The check is a teacher’s red pen: it says nothing when the parts move well. To see every move, good or not, point at a chord. The line over the chord names lists each part from the top down with where it goes, and then the motion of the outer parts. Pointing at G7 in the exercise:

Lineup Osc: top A4→F4 · 2nd F4→D4 · 3rd C4→B3 · lowest F3→G3; outer parts in contrary motion

Its tooltip says each move in words, with the interval spelled in the key, “down a major third from A4 to F4, a skip”. How far a part moves has a name:

And two parts move together in one of four ways:

Good part writing moves the inner parts mostly by step or holds them, lets the bass leap from root to root, and keeps the top part and the bass in contrary or oblique motion, because the ear follows those two most. Parallel thirds and sixths are welcome anywhere: they sound sweet and keep the two lines apart.