Augmented sixth chords are a group of chords that include the Italian, French, and German variants, each with specific characteristics and historical naming conventions.
These chords typically contain the scale degrees 6^ and 4^.
Italian Augmented Sixth Chord (It+6)
Composed of scale degrees 6^, 4^, and 1^.
Forms the basis for the other two types of augmented sixth chords.
French Augmented Sixth Chord (Fr+6)
Builds upon the Italian augmented sixth by adding scale degree 2^.
This results in a chord composed of 6^, 4^, 1^, and 2^.
German Augmented Sixth Chord (Ger+6)
Adds scale degree 3^ to the Italian augmented sixth.
Contains the notes 6^, 4^, 1^, and 3^.
Enharmonic German Sixth (EnGer+6)
An enharmonic variant of the German augmented sixth.
Respell the 3^ as a 2^ for resolution purposes.
Resolves only to major II46.
Does not occur in minor keys.
Historical Context
The naming conventions of Italian, French, and German sixths do not have a clear rationale, yet they are consistently used in music theory.
Historically, these names have persisted despite being described as somewhat arbitrary in sources such as the 1964 Harvard Dictionary of Music.
Visualization
Figure 21.2.1 illustrates the three types of augmented sixth chords in both major and minor keys.