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.
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.
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.
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:
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 root or fifth of the chord. Avoid doubling any note with an accidental added or a tendency tone.
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: