Principles of Part Writing

The music of the common practice period did not appear overnight. The principles that govern it were the result of almost 1000 years of scientific research and artistic experimentation. One of the principles is that of equal emphasis on both harmony (vertical) and line (horizontal). The process of writing music that addresses both of these aspects is referred to as voice leading or part writing (these two terms are used interchangeably). Four-part choral music is most often used to demonstrate and teach voice leading, since it addresses most of the problems, methods, and principles for writing for more or fewer voices. Four voice choral part writing is often referred to as SATB (Soprano, Alto, Tenor, Bass) part writing. Although all parts follow the smooth, melodic principles discussed in the previous section on melody, the issue of contour is usually reserved for the soprano alone.

Ranges

The following are the ranges allowed by most theorists for each voice. Although it is certainly possible for good singers to sing beautifully beyond these ranges, it is helpful to have a rather fixed limit for each voices range to effectively study this craft.

When writing parts, it is important to always keep each voice within its range, and also not to allow voices crossing (when a higher voice becomes lower than a lower voice). Voice crossing blurs the distinction between the parts, especially when played on a keyboard instrument. The following is an example of voice crossing. Notice how when the voices cross in this example, the first two beats will sound as though the alto and tenor repeat the same pitches.

Voice overlap is when a line crosses above or below a pitch recently sounded by another voice. Although this is not voice crossing, when the notes are only one to three beats apart, the ear may still hear the voices as overlapping, and the independence of line is diminished. Although Bach and many other great composers occasionally wrote overlapping parts, there was always a good, melodic reason. So, unless there is a compelling compositional reason, this error should also be avoided.

Types of Motion

By definition, two or more voices can move in only four basic types of motion:

1) Parallel Motion - both voices move in the same direction by exactly the same interval. In this example, both voices move down by a M2.




2) Similar Motion - when both voices move in the same direction, but by different intervals. In the following example, the top voice moves up by a M2, while the lower voice moves up by a P5.




3) Contrary Motion - the voices move by any interval in the opposite direction.




4) Oblique motion - one voice moves in any direction by any interval while the other remains on the same pitch, not moving at all.

Part Writing Procedures

In order to achieve a level of independence of line for all four parts, parallel motion of Perfect Octaves, Perfect Fifths, and Unisons should be avoided. Since these are the lowest intervals on the overtone series, when two or more voices move in parallel motion in these intervals, they can blur together and sound like only one voice (in the case of unisons, they actually become one voice). Parallel motion by thirds, fourths and sixths is acceptable and often desirable, however, two voices should not move in parallel motion for several beats in succession. This would reduce their independence. To most easily avoid part writing errors, the following procedure is very helpful:

1. Stay within the ranges of the voices. If you notice that a voice is getting toward the limit of its range, try to move in the opposite direction if possible.
2. Use only notes that are a part of the harmony. Non-Chord Tones will be discussed later.
3. Keep common tones when possible. Often there may be a pitch that is a part of adjacent chords. Since parallel motion is to be avoided, avoiding motion of a voice all together eliminates that possibility altogether.
4. Move the other voices in contrary motion to the bass whenever possible. This one simple step eliminates most part writing errors and creates greater contrast between the melody and bass.
5. The 7/7 rule: scale degree seven (the leading tone) should resolve to tonic (always when in an outer voice), unless a part of a line proceeding down from tonic to dominant, (i.e.: 1-7-6-5) and the seventh of a chord should always resolve down by second.
6. Resolve chromatic alterations (notes with added accidentals) by step (the interval of a second) in the same direction. For example, an added flat lowers the pitch, so it should resolve down by step. Added sharps raise the pitch, and should resolve up by step.
7. Finally, check each voice for undesirable parallel motion between voices. Recheck the ranges for voice crossing and overlap. Just as good drivers look twice before crossing an intersection, good part writing students double check their work.

Doubling

When writing triads, there are only three pitch classes but four voices. This means that two or more voices will have the same pitch class. This is called doubling. When you have a choice, try to double the pitch in the bass or the soprano, except when it is scale degree seven or a chromatically altered pitch. Since these must resolve a certain way, if they are doubled, both voices will move in parallel unison or octaves.

If you follow these basic steps, most part writing errors may be avoided. As rigid as they may seem, you often will have several choices yet available. The following example demonstrates these principles:

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