Classical augmented sixth chords

Italian 6th chord

a.) in first inversion

b.) flat 6 - 1 - #4

c.) double the tonic

d.) resolves to a 1 6/4 chord

German 6th chord

a.) flat 6 - 1 - 3 - #4

b.) resolve to a 1 6/4 or a V chord

French 6th chord

a.) flat 6 - 1 - 2 - #4

What are special about these chords ?

Why does classical composers use these chords?

How are there chords used ?

the chord is an Italian sixth; when an augmented fourth is added above the bass, the chord is a French sixth; while adding a perfect fifth above the bass of an Italian sixth makes it a German sixth (the etymology of all these names is unclear). All usually have the flattened sub-mediant (sixth degree of the scale, A flat in C major, for example) as the bass note -in this case, they tend to resolve to the dominant.

Augmented 6th Chords:

The name "augmented 6th chord" refers to the interval of an augmented 6th between scale degrees b6 and #4.

The augmented 6th chords are a type of chromatically altered chords like the secondary dominant, secondary diminished 7th, and the Neapolitan 6th chords. Augmented 6th chords arise as a result of voice leading; there are no real roots. The b6 scale degree in major (or scale degree 6 in minor) occurs most often in the bass. All the chromatically altered notes should resolve in the direction of their alteration; b6 resolves down to 5 in the bass and #4 up to 5 in an upper part.

The augmented 6th chords often occur near cadences. They usually resolve to the tonic six-four and then to the dominant, especially the German augmented 6th chord, to avoid parallel 5ths.

The four types of augmented sixth chords are summarized as follows:

alian 6th

Major: b6,1,#4,1

Minor: 6,1,#4,1

French 6th

Major: b6,1,#4,2

Minor: 6,1,#4,2

German 6th

Major: b6,1,#4,b3

Minor: 6, 1,#4, 3

Doubly Augmented 6th

Major: b6,1,#4,#2

minor: Not used