Ultimately, harmonizing a melody is a matter of personal taste. Nevertheless, although you have some leeway in the selection of chords, a certain standard of musical communication, known as style, prevents you from exercising complete freedom.
Harmonizing a Chorale Phrase
The following general principles will guide our choice of chords for a Chorale harmonizations as much like those of Bach as possible:
1. You must use half (I–V, IV–V, or ii–V) or authentic (V–I) cadences for the final two notes of each phrase.
2. You should use circle progressions throughout in each phrase. Circle progressions are more often longer and more abundant near the cadence than at the beginning of the phrase.
3. Harmonize each melody note with one chord. It is possible to repeat chords occasionally, but adjacent repeated chords are usually in different positions to provide melodic motion in the bass.
4. Employ first-inversion chords and nonharmonic tones to make a smoother (stepwise) bass melody.
5. Shape the bass line carefully to make it a singable melodic line. Use the principles of species counterpoint as a guide to constructing bass melodies. However, remember that the bass usually has more leaps than the other voices.
6. Avoid overuse of ascending-third and descending-second progressions.
Harmonizing Folk and Familiar Melodies
Many of the suggestions for harmonizing a chorale melody also apply to folk and familiar melodies. The most important difference is the harmonic rhythm. The following suggestions for harmonizing a well-known melody are in addition to those given for chorale melodies and are related primarily to harmonic rhythm.
1. Harmonic rhythm is important in establishing a clear meter. For this reason, chords usually change on the first beat of a measure. In 4/4 and 6/8 a second chord change often occurs in the middle of the measure. In 3/4 a second chord change can occur on the third beat.
2. Two melody notes that skip are usually part of a single chord. Look at skips as opportunities to determine the implied chord. If the two notes that skip do not fit into a single chord, or if the harmonic rhythm will be thrown off by harmonizing them with one chord, change the chord.