The four voices in SATB writing have different characteristics. In fact, 4-part music has three separate layers. The topmost layer is the tune, found in the soprano part. The lowest layer is the bass line or harmonic foundation, and the alto and tenor work together to fill in the inner harmony parts.
Of these three layers, the top layer is what the ear focuses on the most, followed by the bass line, and the inner harmony parts are much less noticeable as individual tunes. SATB is normally written in a vocal style, rather than instrumental.
Each of the three layers has different characteristics, and different voice leading guidelines.
Soprano (Tune)
The soprano part needs to be tuneful and musical. To create a tuneful part, stick to mostly conjunct motion, but don’t be afraid to include one or two leaps to make the tune more interesting. Repeated notes can also be used, but keep them to a minimum, especially in a short phrase. As well as avoiding individual repeated notes, try to avoid repeated patterns or groups of notes.
Here is a tune which works well. It contains mostly conjunct motion, with three leaps, and two pairs of repeated notes.
This tune is less interesting, as the same pairs of notes are repeated too many times and there is no disjunct motion at all.
This tune doesn’t have enough conjunct motion in it, so it sounds like a series of broken chords. While it sounds quite nice, it’s not an appropriate style for a vocal SATB part.
This tune sounds disjointed because the disjunct motion has not been treated in the correct way. There are leaps in the same direction where the notes do not belong to the same chord, and there is an unresolved diminished 5th in bar 3. This one would be considered grammatically incorrect in an exam.
Bass (Harmonic Foundation)
Bass lines normally contain more disjunct motion than soprano parts, in particular leaps of a 4th, 5th or octave are quite common. Leaps of a 6th or 7th tend to be quite rare though, and are best avoided in harmony exercises.
The notes in a bass line dictate which inversion of a chord is being used, so lots of thought needs to go into choosing bass notes. In particular, second inversion chords need to be avoided apart from in places where they are allowed, such as the cadential 6/4.
Here is a typical bass line. There is quite a lot of disjunct motion, but a little conjunct motion too.
Inner Parts (Harmony)
The alto and tenor parts in SATB style should move as little as possible. If it is possible to repeat a note, then that is the best option. Otherwise, move by the smallest distance possible. It is usually possible to write an inner part without needing any interval larger than a 4th.
A typical alto part will look like this:
Tenor parts in SATB can be written quite high on the stave. It is not unusual to see up to three ledger lines, going up to G above middle C.
SATB Leading Notes
We have already learned that the normal voice leading behaviour of the leading note is to rise by a semitone step, if the chord progression is V-I.
In a 4-part SATB piece, the rules are relaxed a little if the leading note appears in one of the two inner parts: the alto or the tenor. This is because the inner parts are less noticeable to the ear than the soprano and bass parts.
When it appears in an inner part, the leading note can, alternatively, fall by a third to the dominant note in chord I. This means the tonic chord can be written with all three chord notes.
But if the leading note rises, the chord will be missing its 5th: There is nothing wrong with writing a chord like this, but if you are writing in the style of a Bach chorale, you should know that at a cadence, the full triad is always used, and the leading note will fall to the dominant in an inner part.
It’s important not to add a passing note between the leading note and the dominant, because this will draw attention to the fact that the leading note is moving in an unusual way. This is true even if there is a passing note in another part.
The harmonic in interval of a diminished 5th, which exists between the leading note and subdominant note in chord V7, does not need to be resolved inwards when it involves either of the inner parts. However, it must resolve if it is in the outer parts (soprano and bass).
Here, the diminished 5th (B-F) between the alto and tenor moves to a perfect 5th (A-E). This is fine.
