·         The system of sevenths and shells can be the foundation of quite sophisticated keyboard harmony, but it has its limits.  The root note is an anchor, tying the chord directly to the bassline; but like all anchors, it restricts freedom too.

·         You recall that the formula of the major triad is I III V.  A simple way of "slipping anchor" - escaping from the limitations of always playing the major chord in its root/shell position - is to make the bottom note not I but III (Example [10.1]).

·         In conventional harmony this would be the first inversion of the chord.

·         The seventh/shell idea can be quite conveniently adapted to the first inversion as shown in Example [10.2].  In the key of G, the seventh above B, note III in the chord of GΔ, is A, which gives us the shell.  By adding D¹ and F¹# we have made a rootless GΔ7 9.

·         Notice that the notes in this form of GΔ are identical with B-7.  This introduces a general rule for major chords: the Δ7 or Δ7 9 with I as the root and the -7 on III are interchangable.  Example [10.3] shows this for CΔ/E- and GbΔ/Bb-.

·         Try a similar manoeuvre with the dominant seventh chord.  In this case I7 (9) can be substituted by ø (half-diminished - see Step 5) on the third.  Example [10.4] presents C7 (9) and Eø as equivalents.

·         In the case of the minor chord I IIIb V, Δ7 on IIIb will substitute for IΔ7 (9).  In Example [10.5] the relevant C-7 (9) and EbΔ7 voicings are given.

·         Example [10.6] is two rounds of a .../II/V/I/VI/... sequence in the key of C, with simple I position shell voicings (.../D-7/G7/CΔ7/A7/...).  Example [10.7] is the same thing but substituting some III position forms, leading eventually to more complex mixes of voicings, added notes and passing chords.

·         Why not try similar things in different combinations and in other keys.