6/4 Chords as Forms of Prolongation

A 6/4 chord (second-inversion triad) is inherently unstable because the bass note is the fifth of the chord rather than the root or third. This instability means 6/4 chords rarely stand alone as independent harmonies; instead, they serve specific voice-leading functions that can be categorized into three prolongation types beyond the cadential 6/4: the passing 6/4, the neighbor (or pedal) 6/4, and the arpeggiated 6/4.

The passing 6/4 has a bass note that is approached and left by step, filling in a stepwise bass motion through the 6/4: a classic example is I–V⁶⁄₄–I⁶ (or IV–I⁶⁄₄–ii⁶), where the 6/4 lies between two chords with bass notes a step apart. The neighbor (pedal) 6/4 keeps the same bass note throughout — I–V⁶⁄₄–I sustains 1̂ (do) in the bass while the upper voices move out and back — so the 6/4 functions as a double-neighbor embellishment of the tonic. The arpeggiated 6/4 arises when the bass arpeggiates through the pitches of a single harmony, landing on the fifth: I–I⁶–I⁶⁄₄ cycles through do–mi–sol.

In all these cases, the 6/4 chord is understood as a contrapuntal byproduct of smooth voice leading rather than an independent harmonic entity. The bass note and the chords surrounding it reveal the function: if the bass is stationary (pedal 6/4), moving by step through the 6/4 (passing 6/4), or arpeggiated from the same root (arpeggiated 6/4), the chord prolongs rather than progresses. Identifying 6/4 chords correctly prevents misreading of the underlying tonal structure, which is always governed by the root-position or first-inversion chords framing the 6/4.