Now that you know some basic intervals and you understand the concept of inversions, it’s time to continue on our tour. In this installment, we’ll look at another pair of related intervals: thirds and sixths.
Thirds come in two flavors: major and minor. We think of major as sounding happy or sunny and minor as sounding sad or ominous. The third of a chord is what defines it as “major” or “minor’ sounding.
Listen to the difference between these two chords:
A major third is the distance of two whole-steps (four frets) and the minor third is a step and a a half (three frets). Both intervals are close enough together that we can play them on adjacent strings, or consecutively on the same string.
Let’s start by looking at the fretboard shapes for major thirds. Here are the shapes for major thirds on adjacent strings:
If we play a major third on the same string it looks like this:
This is the same shape no matter what string you’re on.
Minor third shapes on adjacent strings look like this:
Played on a single string, a minor third looks like this:
Thirds are particularly useful because they’re the building blocks of chords—a topic we’ll get into later.
The sixth is the inversion of the third. Just as there are major and minor thirds, there are also minor thirds. We can play sixths either on adjacent strings, or by skipping a string. Here are the shapes for major sixths on adjacent strings:
A minor sixth is a half-step (one fret) lower than a major sixths, so the shapes for those look like this:
We can also skip a string in between. Major sixths look like this:
Minor sixths look like this:
Sixths can have a real country sound to them:
Now let’s look at the theory side of things. Let’s start by referring to our A major scale chart again:
The sixth degree of the A major scale is F♯. Now let's look at the F♯ major scale:
Now you’ll notice that A doesn’t appear in this scale. The major third in F♯ is A♯. This means that A is the minor third above F♯ since it’s a half-step (one fret) below the major third. So the rule here is that the inverse of a major third is a minor sixth.
The opposite relation is also true: the inverse of a minor third is a major sixth. Let’s work through another example. We know that major third above A is C♯, so the minor third would be C. If we look at the C major scale, we see that A is the major sixth above C.
With all this in mind, let’s expand the inversion chart from the last post:
On the fretboard, the inversions looks like this:
Thirds and sixths are still pretty consonant, but they are “thicker” sounding than fourths or fifths, especially on a distorted electric guitar. It doesn’t mean you can’t play them, you just need to judicious when you do. Played together, thirds sound muddier down low than they do further up the neck. Compare the sound of the first measure below with how the second measure sounds:
You can also invert thirds into sixths which spreads the notes out a bit and cuts down on some of the muddiness. Compare the sound of these two measures:
We have one more pair of intervals to look at after this. Once you have intervals down, you’re ready to move on to triads and chords—which is where the real fun begins!
That’s all for now. Until next time, keep rocking!