In figured bass, a roman numeral followed by a 6-4 represents a triad in second inversion, because the intervals between the bass note and the upper notes is either a 4th or a 6th.

There are three principal types of 6-4 chord:

• The Cadential 6-4. This delays the following V or V7 chord.
• The Passing 6-4. This connects two chords with a smooth bass line
• The Stationary 6-4. This connects two chords with a static bass line

Click here for an audio-visual example of these three types of 6-4 chord.