Fr+6 occurs occasionally in popular music and jazz. The symbol used calls for a Mm7 chord with a lowered 5th. In the key of d minor, Fr+6 would be symbolized as Bb7(b5)
The German Augmented Sixth Chord (It+6 plus minor mode 3)
May be thought of as an It+6 with the addition of a minor mode 3 (chromatically lowered if in a major key)
When the Ger+6 moves directly to V, parallel 5ths are apt to result, because the ear is distracted by the resolution of the interval of the +6, the parallels are not so objectionable, and they may occasionally be encountered
Composers usually manage either to hide the parallels through anticipations or suspensions or to avoid them through the use of leaps or arpeggiations
A simpler resolution to the problem of the parallels is to delay the V through the use of a cadential six-four
The last Ger+6 in example 22-13 spelled differently from the others, although it sounds the same (A#=Bb). This is a common enharmonic spelling of the Ger+6, used in the major mode only, when the Ger+6 is going to I
The reason for the use of enharmonic spelling is more for the eye than for the ear: A# to B♮ looks more reasonable than Bb to B♮ because we usually expect raised notes to ascend and lowered ones to descend
Ger+6 chords are encountered frequently in lead-sheet symbols, where they are indicated as a Mm7 chord on the minor sixth scale degree, as in C: Ab7-G7, which represents C: Ger+6 – V7
Jazz theory explains the Ab7 as a triton substitution; that is, the Ab7 is a substitute for a D7 (V7/V) chord. Their roots are a tritone apart, and the two chords share a tritone: C-Gb in the Ab7 and C-F# in the D7
– V
