3rd Rail Chord Symmetries (Part 1)

Here's an set of patterns which make clear use of the 3rd Rail symmetry to simplify navigation. We can think of 7th chord arpeggios as spirals radiating around a central interval. For instance; the central interval (3rd–5th) in a minor 7th chord is a major 3rd. If the chord is rooted on the 4th string and played as a series of 3rds, the central interval will span the 3rd Rail and the overall shape of the chord will be perfectly symmetrical – therefore, easy to visualize. In Fretography this is the concept of 'centered chords'.

This diagram illustrates the basic principle. Starting with a 3rd interval as the center of the chord, we extend the chord an additional 3rd in both directions, so a 3rd becomes a 7th, then an 11th;

The chords we will study in this exercise are mostly arpeggios, which are chords that are played note by note rather than strummed. Knowing your arpeggios can really enhance your improvisations and your playing overall. Playing a chord as an arpeggio is necessary on the guitar when some of the chord's notes are on the same string and so can't be played at the same time. This is sometimes referred to as a "broken' chord. (We looked at some rudimentary arpeggios in a previous post.)

Below is the A minor 7th arpeggio of the key of C, rooted on the 4th string;

The notes  are shown in descending order on the staff because it visually follows their layout in the fretboard diagram.
Play the notes in ascending and descending sequences.

See how the two minor 3rds within the chord radiate out from the central major 3rd. Now let's look at what happens if we add notes 3rd above and below this chord;

Italic numbers indicate fingering. Fret positions are indicated by bold non-italic numbers. 
Original 7th chord tones are black, added notes are red.

The symmetry is clear. Of course, we can choose to make F the root of this new set of notes, in which case you are looking at an F major 7th sharp 11 arpeggio. But, for our purposes, we will treat the F and B as extensions of the original A minor 7th chord. Play this arpeggio slowly – ascending and descending – using the indicated fingering.

There are three minor 7th chords in any major key. They are rooted in the 2nd, 3rd and 5th scale degrees (D, E and A in the key of C). Though all three of these chords have the same interval structure, their positions in the key give rise to a different set of extended intervals for each.

Below is the extended minor 7th arpeggio rooted on the 3rd scale degree. Notice the same geometry of the central minor 7th chord (EGBD);

This is an E minor 7th, with diatonic 3rd extensions above and below. The C is a major 3rd below the root, and the F is a minor 3rd above.

There's one more minor 7th chord in the key of C. It is rooted on the 2nd scale degree, and it is D minor. The central minor 7th structure (DFAC) is the same as the two previous examples, but notice that it's the geometric and intervallic opposite of the iii chord – E minor.

Below are the three extended minor 7th arpeggios side by side. The vi chord is placed in the middle to emphasize their overall geometric symmetry. Remember that these forms will be in different fret positions in each key but will always retain their relative geometry based on their scale degrees.

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Let's look at these chord forms as they are actually arrayed on the fret board.

Below you see the minor 7th arpeggio rooted on the 2nd scale degree. This time the chord is shown in its natural position on the fret board;

You can see that the geometry of this chord is the opposite of the previous example (the iii chord – E minor). 

Here's the E minor again, with the whole fret board shown around it;

... and the extended A minor 7th;

Remember that these chords are extended versions of these minor 7th chords;

Here are all three extended minor 7ths in their actual positions in the key of C;

... and here's the same set of extended minor 7ths in the key of G;

Next we'll look at the four remaining 7th chords in the Key of C; C major 7th, F major 7th, G dominant 7th and B minor 7 b5.

All contents of this blog are © Mark Newstetter