Doc Snow has been an online writer for over seven years. He's a lifelong musician who loves to record his own compositions.
This article investigates the patterns governing triads in second inversion. Here, you will learn to navigate the tricks and traps with video examples and interactive practice questions! Let's get to it!
It probably sounds as though second-inversion triads--also known as 'six-four chords'--are a bit different from root position and first-inverrsion triads. If so, then the impression given is entirely correct. Traditionally, chords were classified by the intervals they contained, reckoning things upward from the bass tone.
According to this view, root position triads were the most stable, since above their bass tones one would find a perfect fifth and either a minor or a major third. (Diminished and augmented triads are considered dissonant in this position, precisely because their chordal fifths are not the stable perfect fifth, but the decidedly unstable diminished or augmented fifth.)
Less stable are first-inversion triads; they contain a minor or major third, just like the root position triads, but substitute a sixth for the fifth. Both intervals are consonances, though the sixth is less stable than the fifth. Still, first-inversion triads may be used quite freely, as we have seen.
By contrast, second-inversion triads are characterized by an unstable fourth, as well as the more stable sixth. That fourth above a bass tone had, by the time when chordal theory was coalescing, been treated as a dissonance for many centuries. Therefore, the second inversion triad took on a somewhat dissonant quality too.
That does not mean it is not a useful sonority, however. It just needs a little care to use appropriately. We'll examine in turn the normal ways in which sixth-four chords appear in common-practice music.
Alternating and Arpeggio Six-Four Chords
These two patterns are closely related, and operate on the same psychological principle. In both cases, the root position voicing of the triad occurs first--and very often on a strong beat--and is then and only then followed by the second-inversion voicing. This sequence allows the instability of the six-four to be subsumed in a larger, and stable, context. So the second-inversion triad can freely follow the root position voicing of the same chord--even, in a pinch, the first-inversion voicing. But the reverse does not work.
So what differentiates the alternating and arpeggio six-four chords? In a word, the melodic pattern of the bass. Here is how the alternating six-four works, as shown in a lead sheet for the American folk song, "John Henry."
Note how the bass works in the first measure. Although the entire measure falls within a G major chord--see the chord symbol above the treble staff?--the bass alternates between root (beats one and three) and fifth (beats two and four) of the triad. That is the origin of the term 'alternating.' Note, too, how prevalent this pattern is throughout the excerpt--it is not uncommon for this style of bass to continue for long stretches of music.
By contrast, here is an example of the arpeggio six-four--the setting of "Auld Lang Syne" which we have examined twice before: