The arpeggio 6-4 is another one of the few progressions where you can use a second inversion chord. (Other progressions with 6/4 chords that you should already know are the cadential, passing, auxiliary and pedal 6/4s. These were covered in earlier grades.)
Arpeggios, or broken chords, are a common way of writing chords in piano music. Instead of playing all the notes at the same time in a block chord, the chord notes are played separately.
Arpeggio chords should be treated in the same way as their block-chord counterparts, when you are considering voice-leading, consecutives, or any other rules of harmony. The inversion of each chord is usually fixed by the lowest note that is played in each group.
Here is an example of piano chords played in arpeggio style from Mozart’s Piano Sonata in C major, K545. This particular four-note pattern is also known as an “Alberti bass”.
In bar 1, the lowest note is C in a C major chord, so the chord is in root position. In bar 3, it is an F major chord in second inversion. This is an arpeggio 6/4 chord.
The composer’s choice of inversion will depend on a number of factors, including the tune created by the bass line itself, voice-leading in the bass part, the harmony produced between the combined bass and the melody parts, the need to avoid consecutives, and how the chord fits under the player’s hand.
If Mozart had used a root position F major chord here, there would be consecutive 5ths. Root position consecutive arpeggio chords are never used for this reason. The 5ths are easier to see if you view the chords as block chords:
If Mozart had used a first inversion chord, the combined bass and melody would be a bare octave on beat 1, with a doubled third of the triad. This version does not break any rules of harmony, but it sounds less beautiful than the second inversion chord because it is 3rds and 6ths which create warm sounding harmony.
