·         So far all the chords discussed have been tertian, built from notes a third apart.  Now we look at quartal harmony - chords in fourths.

·         Even though it isn't in the least dissonant, the interval of a fourth is strangely difficult for the ear to make sense of.  Hindemith suggested that when two notes a perfect fourth apart are sounded, the ear recognises the upper of the two as the root!

·         This strengthens the case for the lydian, in which note I is endorsed as the keynote because its main rival in the major scale, IV, is destroyed by sharpening it.

·         It also means that quartal major chords built on I are not very satisfactory.  The sharp fourth interval (the tritone - see Step 3) in the foundation of such a structure has the strong flavour of a dominant seventh, and although there are certainly contexts in which the root position lydian by fourths (I IV# VII) is usable as a form of IΔ, they are rather specialised.

·         Quartal Δ chords can, however, be very effectively built on III.  So in the key of C, a Δ chord by fourths would be E A D¹ G¹ C² (Example [11.1]).  In conventional notation, this is equivalent to C6 9.

·         E A D¹ played in the left hand can function as a kind of quartal shell, expressing the special flavour of chords in fourths while allowing harmonic and melodic freedom in the right hand.

·         The quartal shell on VI - A D¹ G¹ in CΔ - is also an effective voicing.

·         If you want to thicken up the textures of these rather sparse structures, add the next fourth in the series (above or below the shell) to the middle of the chord.  Thus E G A D¹ or E A B D¹ (Example [11.2]) and A C¹ D¹ G¹ or A D¹ E¹ G¹ (Example [11.3]), containing a cluster of notes a second apart, are richer, fuller forms of quartal C6 9 chords.  For more about cluster voicings, see Step 17

·         The lydian is a perfectly suitable scale for improvisation against such chords; but notice that the notes C D E G A are prominent in the chord structures associated with C quartal.  These notes make up the five-note, or pentatonic, scale (everyone knows F# pentatonic because it's the black notes on a piano).

·         Pentatonic scales are characteristic of the music of the Far East.  So some of the special texture of quartal major chords comes from the exotic, faintly oriental flavour of the scales they imply.

·         Work out III and VI-position quartal 6 9 chords for all keys and their associated pentatonic scales.