It’s a chord created around the interval of an augmented 6th. It’s often based too on the flattened 6th degree of the scale – major or minor (a flattened submediant).
There are three different types – the Italian 6th, the French 6th and the German 6th – the German version sounding like a ‘dominant 7th’ chord.
To understand their formation, work out first and foremost what the interval of an augmented 6th actually is.
Then, look at the three different types of augmented 6th chords.
Lastly, it’s easier to understand them and their context if the musical ear can detect the resolution each time. The uppermost note moves upwards one semitone, whilst the lowest note moves down a semitone, like this:
augmented 6th interval
the ITALIAN 6th chord resolving onto chord V in C major
the FRENCH 6th chord resolving onto I64 in C minor
the GERMAN 6th chord resolving onto I64 in C major
This is a very useful video to see how classical and contemporary composers have used this aug. 6th chord:
Chords of the augmented 6th
A chord deriving from the interval of an augmented 6th. There are THREE types of augmented 6th chords – the Italian 6th, the French 6th and the German 6th.