A chord progression can be viewed in two different ways. On one hand it can be viewed as a succession of vertical chords, while on the other it can be viewed as a combination of separate horizontal melodic parts. Most music of the eighteenth and nineteenth centuries is conceived on a harmonic basis of four-part writing. Larger numbers of apparent pans, as for instance in an orchestral score, are usually the result of reduplication of one or more voices in a basically four-pan texture. This means vertically four factors in each chord, and horizontally four different melodic voices. This observation is very important for the study of harmony, because it means that when writing a harmony, we need to consider not only the chord progression itself, but also the melodic movement of each of the individual parts, which serve to connect the chords logically together. This feature—the melodic movement of individual parts within a chord progression or the sustained notes obtained by exploiting notes that the chords share in common—is called voice leading. Also knowledge of inversions, doubling, and spacing (see below) can give your chord progressions spread, coherence, and logic.
Four Kinds of Part Motion
The study of harmony is concerned with the principles underlying the construction of the common basis of four-part writing. The exercises are ordinarily worked out in four voices. The ten “voices” does not necessarily mean that the parts are to be sung. It does suggest, however, that the separate parts proceed with a certain ideal vocal quality which is possessed by all good melodic music, whether intended for human voices or for instruments.
When writing more than one melodic part at the same time, there are four possible types of part motion. The motion of one voice relates to the motion of another in one of the four following ways
Contrary motion creates the greatest contrast between the two voices and helps to give each an individual contour. Each voice is independent of the other in a way that adds to the listener’s interest. Of the remaining types, oblique motion is the next most independent, similar motion less so, and parallel motion least of all. Because there are only two directions, up and down, all four voices cannot be going in contrary motion to each other; thus, the other types of motion are not only permissible but necessary and desirable. In particular, parallel motion in 3rds, 6ths, and 10ths can be among the most useful types of voice leading.
In chorale writing, the voices are divided into four general categories: soprano, alto, tenor, and bass. In the figure below, whole notes indicate the best usable ranges. Black notes represent pitch ranges that should be used sparingly. The greater part of each line will normally lie within the middle of the range rather than at the extremes.
Although not as crucial as the bass tone, which determines whether a chord is in root position or inversion, the soprano tone also exerts a strong influence on the sound of a chord. The reason is that the soprano is usually the most prominent of the four voices. The effect that the soprano tone creates depends on several factors in combination: the interval from the bass, the stability or activity of the scale degree, and the context in which it appears. For instance, the scale degree 1 over tonic harmony tends to give a stable— indeed, sometimes a static—impression; it forms an octave with the bass and is the least active degree of the scale. Thus, this position of the soprano will be most useful at points of articulation. A particularly beautiful sonority often results from placing the 3rd of a triad (especially a major triad) in the soprano. This disposition mirrors the overtone series, in which the 3rd (5th partial) appears above the octave (2nd and 4th partials) and the 5th (3rd partial).
The two outer voices (Soprano and Bass) are of a very mobile nature. They might be likened to a curved line rising and falling in waves. Fifths and fourths are characteristic jumps in the Bass; though this voice is also very partial to scale·-like progressions by steps, mostly in contrary motlon to the Soprano
As the highest, and therefore most exposed, voice, the soprano carries the main melodic line. Most good soprano lines contain a preponderance of conjunct motion, but the inclusion of one or two leaps will help greatly in adding interest and variety to the line. On the other hand, too much disjunct motion (The terms “conjunct” and “disjunct” motion are used to describe the movement of a single line by step and by skip respectively) may keep the line from holding together and may make it difficult to sing.
Because the lowest tone is the crucial member of the chord, the bass voice has the special function of regulating the succession of chords. Bass and soprano lines are interdependent. The bass must make explicit the harmonic meaning of the soprano.
The inner voices are distinctly averse to much motion; they are fitted by nature to retain the common tones of chords. Often they retain one and the same tone through two, three or more bars. The inner voices are best kept at an altitude not too high and should not be to far apart from each other. Where the chord is not in close position, however, they must not approach each other too closely. We must be peculiar to avoid a position of the voices, in which the inner voices are close to the Bass, with the Soprano lying very high. Theory can supply but a very general insight into the nature of the voices.
Inner voices sometimes have a melodic interest of their own, particularly in places where the soprano does not move very much. Their main function, however, is to complete the tones of the chord framed by the bass and soprano. The position of the outer voices may limit the melodic possibilities of the inner voices, so that extensive repetition of one or two tones may be unavoidable. Such repetition is not injurious to the total effect if the bass and soprano are good. In general, the inner voices will have a less distinct profile than the soprano and bass will.
When using a chord the different notes of the chord are spaced relative to one another. The voice spacing of individual chords in four-voice textures is said to be either close or open. Chords in close position (no additional chord tone can be inserted between adjacent voices) have less than an octave between the soprano and tenor (a), whereas chords in open position (the upper voices are separated so that a chord tone could be inserted between either alto and soprano, or tenor and alto, or both pairs of voices) have an octave or more between the soprano and tenor (b).
The factors that make the aforementioned effective ways of voicing the chord are the balanced and as even as possible arrangement of the intervals between the lead note and the bass note.
Of course, the notes of a chord don’t have to be confined to standard inversions within a single octave interval. To avoid the clash of the seventh and root notes of a seventh chord, you can always spread out the notes, moving one or the other an octave higher or lower. This creates what is called an open voicing, where similar instruments are spaced a fourth or more apart.
The opposite of open voicing is closed voicing. With closed voicing, similar instruments are spaced a second or third apart. As you can imagine, closed voicings impart a tighter and more dissonant sound, and serve to create a bit of tension within an arrangement.
Intervals wider than the octave are usually avoided between soprano and alto, and between alto and tenor, but are considered quite satisfactory between tenor and bass.
Treatment of Leaps.
Melodies that move primarily from one pitch to another adjacent pitch — that is, moving by seconds and unisons — are said to have conjunct motion. Music that is easy to sing often consists primarily of these smaller intervals. Melodies that move mainly with intervals of a third and larger are said to display disjunct motion.
Disjunct motion gives variety and tension to a melodic line, but can be disruptive if used carelessly. The effect a leap produces depends largely on its size and on whether it is consonant or dissonant. (In context, other factors might be important, for instance whether or not there is a chord change.)
Consonant leaps—upward or downward—occur fairly frequently even in the simplest vocal textures. The smaller the leap, the less it tends to disrupt the continuity of the line. Thus, a leap of a major or minor 3rd interferes least with melodic continuity, especially where it is preceded or followed by stepwise motion. A leap of a 6th or octave, on the other hand, generates considerable tension and should usually be followed by a change of direction, preferably by step. Leaps larger than an octave are not permitted. Large leaps must be used sparingly in a short harmony exercise; although their occasional use creates interest and variety, too many will create a disconnected, meaningless line. The melodic perfect 5th and perfect 4th (as a melodic interval the perfect 4th is always consonant) are more moderate in their effect than the octave or 6th but far more noticeable than the leap of a 3rd. It is best often to change direction after such a leap, but stepwise motion in the same direction, is a good possibility.
Dissonant leaps represent a more advanced stage of complexity than consonant ones do; consequently dissonant leaps are excluded entirely from the simplest vocal styles. In a four-voice chordal setting, certain types of chords—particularly the dominant seventh—and certain harmonic progressions make dissonant leaps logical and attractive. The augmented 2nd, an interval traditionally excluded from four-part chorale settings and figured-bass realizations, should not be used.