Ever deeper into chord formation dives Andy Honeybone. This month compound intervals, which give you better interest (accountancy joke).

Having introduced the five categories of chords last month, I'd like to follow it up with extensions of the basic harmonic schemes.

The table of intervals in the same issue was deliberately kept within one octave for the dual purpose of appearing less off-putting and saving my addled brain. But let it be known that larger intervals are possible — for instance ninths, tenths, elevenths and thirteenths. Obvious questions: why stop at thirteen, and what happened to the twelfths?

First off, intervals greater than an octave span are known as compound intervals. By subtracting seven, you can reduce them to the recognisable simple intervals from which they spring — the note which makes the interval being shifted up a whole octave. Thus a ninth is nought but a second plus an octave.

But a compound interval is only worthy of mention if it adds something within a chord rather than just doubling at the octave a note that already exists in the chord. Hence a twelfth and a fifteenth are dull because they correspond to a fifth plus an octave, and a double octave.

These compound intervals have their own naming schemes for their semitone up/down variants, based on their reduction to simple intervals. For example major ninths (up from major seconds), thirteenths (up from sixths), and perfect elevenths (up from perfect fifths). Chord symbols show the alterations to these intervals by plus/minus or sharp/flat prefixes, eg G7(-9) = G7(b9) = G, B, D, F, Ab.

Let's see the effect of adding these intervals to our basic chord stock. Once again, major is first under the microscope; though perhaps not the best place to start because only a major sixth, a major seventh or a major ninth can be added without breaking the all-major and perfect-interval rule for strictly major chords.

The sixth (eg E G#, B, C# = E6) is not a particularly strong chord and apparently owes its existence to four-part horn section writing for big bands. Rather than double a note whenever a major chord was encountered, these canny chaps invented the sixth to preserve the depth of harmony. Still argued as a non-chord because it is an inversion of a minor seventh, the sixth is a vastly underused colour.

The major seventh (B, D#, F#, A# = Bmaj7) on the other hand is vastly overused. Just to hear a sustained major seventh chord these days often means that swirling dry ice and pretentiousness are not far away. And to think that 80 years ago, Debussy was clipped round the ear for their introduction (major sevenths, not dry ice effects).

A slight chord symbol digression here. When a chord is described as a major seventh, the major prefix relates to the seventh not the third. A minor chord may contain a major seventh. Here's the cruncher. When a chord is described as a minor seventh, the minor prefix does relate to the third. The seventh is already minor by convention unless specified. OK?

So what does a major ninth (D, F#, A, C#, E = Dmaj9) offer? A twee and less hackneyed major seventh. Although a terrible generalisation, it could be said that the major ninth note is best left as an interesting passing vocal harmony.

You may have detected a certain jaded approach to major chords on my part. They are a problem in that there's little you can do to disguise their presence and they can bring harmonic invention crashing back to the banal.

The hip way out of this straight-jacket is to resort to suspended and parallel fourth chords which avoid classification as either major or minor. More of this in coming issues: until then may I offer the 6/9 in consolation (Ab, F, Bb, C, Eb = Ab69). It's a rather open chord which doesn't seem too happy to perch on its root bass note.

And so to the minors. There is one train of thought which proposes that pure minor chords with all major or perfect intervals except the third should be shunted into a class of their own, away from those nasty minor seventh types. A pure minor grouping would include minor sixths, minor six-ninths, minor elevenths, and minors with major sevenths (Bb, Db, F, A = Bb minor maj7). (Note that a natural sign is sometimes used to denote a major seventh.) I reckon this to be nit-picking: the remaining minor ninths (eg Bb, Db, F, Ab, C = Bb minor 9), and the elevenths containing a minor seventh, might as well be lumped into the same group under the general heading "minor".

In use, the minor ninth is an easy-going general replacement for your average minor seventh. If you pick an inversion where the minor ninth is "folded" into a second slap bang next to the minor third (eg A, G, B, C, E = A minor 9), the closeness of those two notes brings out an intriguing tension.

In the root configuration, as in the Bb minor 9 example above, there is the danger of sounding like something from an old Focus LP. If the seventh of this same example is made major — that is Bb minor 9 (maj7) — then you have a good musical question mark on which to finish a suitably tongue-in-cheek piece.

There's just not space to do justice to the dominant seventh class, so I'll hold such delights over to next month along with the diminished and the half-diminished gems. Until then, perhaps you might like to take a furtive look at your guitar chord dictionaries and see what a mess these extended chords are. It has to be said that when the flattened ninth ends up low on the bass E-string, your voicing will leave a lot to be desired. If you can see the funny side of that then this article has succeeded.