Augmented 6th Chords
A pre-dominant, melodic functioning chord that contains the interval of an augmented 6th

All three types contain the following pitches
1. Sharp 4th scale degree (lower leading tone to the dominant pitch)
2. Minor 6th scale degree (upper leading tone to the dominant)
(lowered in major keys; no accidentals needed in minor)
3. Tonic pitch
Italian Augmented 6th adds
No other pitches (typically the tonic pitch is doubled)
French Augmented 6th adds
Second scale degree
German Augmented 6th adds
Minor third scale degree
The interval of the augmented 6th is the distance from the minor 6th scale degree up to the sharp 4th scale degree
Typically, the minor 6th scale degree is in the bass

Occasionally in the German Aug. 6th chord, the minor third scale degree is respelled enharmonically as a sharp 2nd scale degree to avoid parallel fifths in the resolution.

Usually followed by a V or I64
The lower leading tone (sharp 4) resolves up to the dominant pitch
The upper leading tone (minor 6) resolves down to the dominant pitch