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Lesson 5: How to find third interval harmony notes, anywhere on the fretboard

A young boy finds notes on a guitar fretboard

In lesson 4, we looked at the important role that third interval notes play in: 

  • building chords and making them either major or minor
  • creating harmony if you want to play them alongside your melody notes.

In this lesson you'll learn how to find third interval notes quickly anywhere on the fretboard.

We've already looked at how the guitar is an instrument tuned around intervals of perfect fourths and major thirds.

You've also learned that the perfect fourth interval note is usually directly below your root note. So, if I play the note G on the E string, I know that the fretted note below it is going to be the perfect fourth interval (the C note):

An illustration that G is a perfect fourth interval from, and directly below the C note

Let’s just check if that's the case and refresh our memories. Well, the fretted note on the fifth string is the note C. So there should be an interval of a perfect fourth between G and C:

Tonic is G

Flat 2nd is G#

Second is A

Flat third is A#

Major third is B

Perfect fourth is C

If I know that the strings are tuned in intervals of a perfect fourth, I can easily work out where the major third is - it’s just one fret below the perfect fourth.

Locating the third harmony note in relation to the tonic

So you can find the third of any tonic (root) note, by following this pattern, wherever you are on the fretboard:



And remember, when we get to the B string, because it’s tuned at an interval of a third, we would have to move the note on that string half a step to get the same spacing. So on our B string, the third interval note is directly below the tonic note.

Using the octave to find an alternative third interval note

Now, when we played the intro to Blackbird, we found our thirds on the B string, so you may be wondering how that fits in with what I’ve just showed you.

Let me tell you what I mean. 

When we played our A melody note, our harmony note was the note C.

Well, I don’t really have another C note available to me in this position.

The closest C I have is 5th fret, third string, and that’s too much of a stretch.

But if I go to the octave note, the next A in this position, I can use this to locate a C  on the B string.

If you need to find the minor third, you simply move the third note one fret down the neck, towards the nut.

So, you should now know how to find third interval notes quickly anywhere on the fretboard.

Go to Lesson 6: Finding fifth intervals

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A guitar journal will enable you to:
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Photo by Pavel Danilyuk

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