Harmonic substitution—Harmonic substitution consists of changing the qualityof a chord, that is to say altering one or several notes of the infrastructure. Themost common use of this rule produces secondary dominants: in sequencesbased on fifth relations expressed by functional degrees (I-IV-vii-iii-vi-ii-V-I),this consists of transforming any of the chords preceding V (except IV), that isto say either vii, iii, vi, or ii (all chords with minor thirds) into a seventh chord:

Jazz musicians often use different harmonic substitutions to modify the content of diatonicprogressions. The most common harmonic modifications of the Imaj 7 –vi 7 –ii 7 –V 7progression involve the use of secondary dominant chords (V7/X), and tritone substitutions(TR/X 7). Figure 13.5 demonstrates these substitutions using Model III of harmonicrealization.The secondary dominant 7ths A7 and D7 are analyzed with two different Roman numerallabels VI7 and V 7/ii, and II 7 and V 7/V, which can be used interchangeably. The tritonesubstitutions, E≤7 and D≤7, also use two Roman numerals, ≤III7 and TR/VI7, and ≤II7and TR/V7, respectively