We are writing in four parts but triads have three notes. We will double the root of every root position chord. (The root is in the bass when a triad is in root position, so we are doubling the bass note in an upper part.)
Always double the root in root position triads.
To move from one chord to the next, consider the interval that occurs melodically in the bass line between the current chord and subsequent one.
Bass movement of the interval of a 3rd or 6th
In the upper three parts (soprano, alto, and tenor): retain the two common tones and move the other voice by step
Bass movement of the interval of a 4th or 5th
In the upper three parts, do one of the following:
2. Move all of the upper voices in the same direction (each upper voice will move by a 2nd or 3rd to the nearest chord member in the second chord)
Bass movement of the interval of a 2nd
Move the three upper parts in contrary motion to the bass line.
There is one exception, which leads us to the following principle:
Voice leading the deceptive progression
When the bass moves by the interval of a 2nd in the deceptive progression in the minor mode (V–VI in C minor), always move the 3rd of the V chord up by step while the other notes (the 5th and the doubled root) move in contrary motion to the bass.
In this situation, two voices move in an upward direction and two voices move in a downward direction. This results in a doubled 3rd in the VI chord. In the major mode, use this voice-leading solution if scale degree 7 is in the soprano voice (because it is a tendency tone).
The following two examples demonstrate the two possible ways to deal with the deceptive progression in the major mode.
Repetition of the bass note
Repeat the upper three notes or arpeggiate the upper notes to different chord members while maintaining the voicing of a doubled the root, one 3rd and one 5th.
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