- it contains a tritone which is rotationally symmetric (~ +6 transposition)
- a diatonic pitch class set consists of a pentatonic and this tritone pair
- see Mirror key
- two diatonics within a mirror key pair share exactly this tritone pair
- a dominant chord in one key and dominant chord in its mirror key also share exactly this tritone pair
- other diatonic chords do not allow this
- alteration thus naturally allows visiting the other key in the mirror pair
- in jazz theory using a dominant chord from the mirror key is called tritone substitution
- example: IIb7 instead of V7, eg. Db7 instead of G7
- similar is the Neapolitan chord
- compared to IIb7 it has fifth instead of seventh
- eg. - C -> Db ~ Fm6-5