You should use the second inversion of any triad with extreme caution because of its unstable nature. The chord contains the interval of a fourth and cannot be used in the functional way that typifies both root position and first-inversion triads. The second-inversion position of the tonic chord is common, but that of other triads is found only occasionally. You should employ second-inversion triads only in one of the following ways:
Cadential—The tonic 6/4 chord resolves to the V chord at the cadence. Used in this manner, the 6/4 chord is a decoration of the V chord. The bass note is doubled (a).
Passing Bass—The bass note (5th factor) of the 6/4 acts as a passing tone. The passing bass may be found as a tonic 6/4 between the IV and IV6 chords (b) or as a dominant 6/4 between the I and I 6/4 chords (c). The bass note is doubled.
Arpeggiated Bass—The bass note (5th factor) participates in an arpeggiation of the same chord (Figure 9.16d). This usage of 6/4 chords occurs occasionally with triads other than the tonic. The bass note is doubled.
Pedal Bass—Also known as stationary bass or neighboring tone chords, the bass note (5th factor) is preceded and followed by the same tone and is placed between two root positions of the same triad. This type also occurs occasionally with the IV6/4 (e) as well as the tonic (f). The bass note is doubled.