Guitar Lab: Thirds and Sixths

Tertian harmony is defined as chords built utilizing the intervals of thirds. Thirds are simply every other note of the major scale. When these thirds are stacked on top of each other, the largest chord that can be built is a 13th chord.

This approach utilizes all 7 notes of the scale (mode) with the following formula: 1, 3, 5, 7, 9, 11, 13. This resulting 7 note group is called an heptatonic arpeggio. The advantage of knowing your modes in terms of the heptatonic arpeggios is that you can clearly see all the harmonic options that are available. Depending upon the formula for the mode that you're dealing with, certain scale degrees in this general heptatonic formula may be altered with a flat or sharp.

This lesson will take intervals of thirds out of the C major scale and demonstrate their layout on the 3rd and 2nd strings. This pair of strings are the only pair on the guitar that are tuned a major 3rd apart. You'll only be dealing with 2 shapes: the major 3rd where both notes are in the same fret, and the minor 3rd where the notes will be in adjacent frets.

In the previous lesson, you learned the thirds in the key of C on the 3rd and 2nd strings. All the remaining adjacent string groups on the guitar are tuned a perfect 4th apart. The patterns that you will learn in this lesson will apply to all of these string groups. Again you'll only be dealing with 2 shapes: major and minor thirds.

Let's now take this principle of tertian harmony and apply it to the major scale to generate the diatonic triads. As you look at chart number 1, place 3 fingers on every other note in the key of C starting on the 1. The resulting notes will be C, E, G. These are the 3 notes in the I chord in the key of C and they form a C major triad. If you take your three fingers and move each note up to the next note in the scale, the resulting notes will be D, F, A. This is the spelling of the ii chord in the key of C, which is a Dm.

If you continue with this approach, you'll generate 7 diatonic triads which will always follow the chord qualities which are listed in the lower left-hand box of this chart. Memorize these chord qualities with their respective positions: I, IV, and V are major triads, ii, iii, and vi are minor triads, and the vii chord is a diminished triad. The definition of a triad is a 3 note chord built in intervals of thirds which you now know are simply every other note of the scale. Notice how any two triads that are a 3rd apart will share two common tones.

The Ionian mode is another name for the major scale, and the formula will be 1, 2, 3, 4, 5, 6, 7. When these notes are aligned in thirds (the heptatonic arpeggio), the numbers are as follows: 1, 3, 5, 7, 9, 11, 13. Knowing this information will help you organize your thirds based upon the prevailing harmony.

The Dorian mode is the second mode of the major scale and it's formula is: 1, 2, b3, 4, 5, 6, b7. When these notes are aligned in thirds (the heptatonic arpeggio), the numbers are as follows: 1, b3, 5, b7, 9, 11, 13. Knowing this information will help you organize your thirds based upon the prevailing harmony.