Just as writing a smooth, interesting melody is important, a smooth, interesting bass line is also important. Our ears are more able to hear the outer voices (i.e. the highest and lowest) than inner voices. For this reason, special care must be taken when writing bass lines. However, when the bass note of a chord is changed, the inversion of that chord also changes. Section 11: Inversion of Chords showed how to label and identify chords in inversion. This section will discuss how these inversions are used in common practice music.
Music of the common practice is concerned with both the melodic (horizontal) and harmonic (vertical) aspects of music. For this reason, both of these parameters must be considered when writing a bass line. Root position chords are most stable, because the upper notes more easily fit into the overtone series of the bass note. When a chord tone other than the root is in the bass, the chord becomes less stable. However, when only root position chords are used, the bass line becomes rather jumpy and not very melodic. There are also times when chords in root position may cause the melody and the bass line to move in parallel motion (this will be discussed in the following section in greater depth). Root position chords are generally used at the beginnings and ends of phrases, where greater stability is desired. Less stable inversions are found more frequently in the middle sections.
Use of first inversion chords is most common. First inversion chords are only slightly less stable than root position chords. Composers frequently use them when a root position chord would cause an undesired jump in the bass. Compare the following two examples, the first in all root position, the second using first inversion chords.all root position:
use of first inversion chords to smooth out bassline:
Notice that in first inversion chords, the root or the third of the chord is most commonly doubled (doubling is a term used to refer to when two voices are on the same pitch class). First inversion chords also allow the bass line to move in the opposite direction of the root movement. This is a useful and important feature that will be discussed in the following section.
These are the least stable of the inverted chords. Because of historical and traditional reasons, the interval of a fourth above the bass has been treated as a mild dissonance in much of music of the common practice period. Second inversion chords contain this interval and because of this, require special treatment. Second inversion chords are not freely substituted or used, but instead, are used in only four specific forms:
This is a specific and most common use of second inversion triads, found frequently at the end of phrases. It consists of a second inversion tonic triad, followed by a root position dominant chord, which then usually resolves according to form a cadence. Notice how the bass note is doubled and remains on the same pitch class in a cadential 6/4, and the other voices resolve smoothly downward. For this reason, these two chords (I 6/4 and V) are almost always grouped together as a pair, and as such, form a cadential 6/4 cadence.
Passing 6/4 chords occur between a root position and a first inversion chord, and result in smooth, step-wise motion in the bass. Passing 6/4 chords may be used with ascending or descending bass lines.
In a pedal 6/4 chord use, the bass note remains the same, and the 6/4 chord is preceded and followed by the same, root position chord. The bass note of the pedal 6/4 chord is doubled. This is frequently used in the plagal cadence (amen) in many hymns.
Sometimes the composer will give the melody to the bass line rather than the highest voice. However, in these cases keeping the melody intact and recognizable is more important than the uncommon inversions that may result.Melodic use:
Composers also may arpeggiate the bass line in musical practice (see section 11), and as a result, brief 6/4 chords may occur in the middle of these passages. However, since they do not actually resolve, these 6/4 chords are usually not analyzed as such, since they are not functional.Arppeggiated use:
Since a third inversion chord is both an unstable seventh chord and has the dissonant seventh in the bass, it is less commonly used than other inversions. The bass always resolves down by step (as will be discussed in the following section).