Spacing of chord tones applies to the interval distance between the soprano and tenor.
- Close structure (C): less than one octave between soprano and tenor. In close chord structure, upper voices (S-A-T) will not skip any chord tones from voice to voice.
- Open structure (O): more than one octave between soprano and tenor. In open chord structure, upper voices must skip chord tones between voices.
- Open/Octave structure (O/O): an exact octave between soprano and tenor, most commonly used for first inversion major and minor triads, where you double the chord tone in the soprano with the tenor voice.
The spacing between pairs of upper voices (S-A and A-T) must be an octave or less to maintain the feeling that the voices blend together to provide harmony.
Since triads are three-note chords, and we will use a four-voice texture, you will need to double one chord tone in each triad. Doubling rules are not absolute, but we generally follow certain conventions.
- For major and minor root position triads: double the root of the chord (bass plus one other voice).
- For first inversion major and minor triads: double the chord tone in the soprano, usually with the tenor voice for Open/Octave structure.
- For second-inversion major and minor triads: double the bass (the chordal fifth).
- For diminished triads in first inversion: double the bass (the chordal third). Note that diminished triads are almost always written in first inversion.
- Seventh chords have four chord tones and do not require doubling. It is possible, however, to omit the chordal fifth and to double the root.
- In a few situations (typically the end of phrases), it is possible to omit the chordal fifth for major and minor triads and to triple the root. Never omit the chordal third of a triad.
Note that this list of common doublings does not include augmented triads. This omission is because augmented triads are seldom used in common practice harmony.