Instant Harmony: Just Add Thirds

This was the 10th Fret Science YouTube video, uploaded on October 9, 2023. It was my first video to include live-action footage, and OMG, it is hard for me to watch this today. Still, as of December 2025, this video has 89k views and added ~1600 new subscribers, so I'm calling it a win.

Original video description: Harmonized solo guitar lines elevate some of the greatest guitar music -- everything from The Beatles to Nuno Bettencourt. This video teaches the easiest way to create these lines, with just enough music theory to get by and pointers to more in-depth explanations as needed.

Instant harmony: just add thirds

It’s easy to create on-the-fly harmonized lines if you know these five things:

1. How the major scale sits on one string
2. Which scale degree your finger is on
3. Which scale degrees map to major thirds and which map to minor
4. How to find major/minor thirds on a neighboring string
5. How to move around inside the scale.

A major scale is made up of a fixed sequence of whole- and half-steps. Shown below is a C major scale played on the B string. Note that there are half-steps between the 3rd and 4th scale degrees and the 7th and 1st scale degrees. All other intervals are whole steps.

Diatonic thirds are created by playing any note from the scale along with the note two scale degrees higher. The scale repeats and continues forever, so we have seven possible combinations: 1+3, 2+4, 3+5, 4+6, 5+7, 6+1, and 7+2.

Of these thirds, the ones built on the 1st, 4th, and 5th scale degrees are major thirds (4 fret spans) and the ones built on the 2nd, 3rd, 6th, and 7th scale degrees are minor thirds (3 fret spans).

When navigating the scale, pay close attention to the locations of the 1st, 4th, and 5th scale degrees, because you’ll be building major triads there, and minor ones everywhere else. The locations of the half-step intervals can be useful waypoints for navigation.

There are multiple ways to play major and minor thirds, and it’s worth learning fingerings for several of them. The video focused on these three:

Here and in the following diagrams, major thirds are connected with a red line, and minor thirds are connected with blue. The stars in the diagram above represent the string and scale degree where you keep your focus while building an on-the-fly harmonized line. Note that the first set of shapes does not straddle the “warp” between the G and B strings, but the other two sets do. Also note that in the second pair, the stars are on the upper string. For the full set of fingering options, see the diagrams on the next page.

Here are some examples of harmonized scales using these fingerings. This one is a C major scale on the B string, with the thirds played on the E string. To move between these shapes efficiently, keep your index finger on the E string at all times, and use your middle finger to play major thirds and your ring finger to play minor thirds:

This example is a C major scale on the B string, with the thirds played two strings lower on the D string. To move between these shapes efficiently, keep your middle finger on the lower string, and use your index finger for major thirds and your ring finger for minor thirds:

This example is a B♭ major scale on the A string, with the thirds played three strings higher on the B string:

The following diagrams summarize all of the practical ways you can finger thirds and sixths. For purposes of this lesson and the accompanying video, the red note is always the focus for creating the harmonized line. You will need to figure out which scale degree it is, and make your third major or minor as needed. This gets much easer with a little practice.

Notice that in these diagrams, the fingerings in the top row in each section do not straddle the “warp” between the G and B strings, but ones in the bottom row do. Between each pair, the portion of the fingering that crosses the warp is shifted by one fret.

Also notice that the black note in each minor third is exactly one fret lower than the black note in the corresponding major third diagram.

As I explained in the video, one of my favorite “tricks” is to use spread thirds around a captured open string that acts as a pedal tone. This is what Paul McCartney did for much of “Blackbird”, which I think is one of the most beautiful guitar parts ever written.

In these examples, you’ll focus your attention on the lower string in the spread third, and you can capture the sound of any mode by taking the note from the open string and assigning the scale degree corresponding to the mode you want to that note on the lower string. Remember that 1=Ionian/major, 2=Dorian, 3=Phrygian, 4=Lydian, 5=Mixolydian, 6=Aeolian, and 7=Locrian (there’s a whole separate video on that).

This is easier to explain visually, so here are a few examples from the video. If we capture the open G string as a pedal tone and think of the G note on the A string (10th fret) as the second scale degree, we’d be playing G Dorian.

If we shift the scale so that the G note on the A string (10th fret) is now the fifth scale degree and continue using the open G string as a pedal tone, we’d now produce a G Mixolydian sound.

And if use switch to using the open B string as the pedal tone and map the B note on the D string (9th fret) to the fourth scale degree, we’d produce a B Lydian sound.

There are a ton of combinations to try, and there are a huge variety of sounds you can get with this simple technique, so play around and see what you like!

I’ve found that one of the best strategies for identifying which scale degree your finger is on is to visualize the scale using fragments of the three-notes-per-string (3nps) scale shapes. The fingering fragments fit together like puzzle pieces both horizontally and vertically. (I have a whole separate video on that!)

But you can (and should!) also learn the full major scale along one string:

One final trick I demonstrated in the video was using a 3rd interval fingering on the E and G strings as a “stab” to highlight an open position major chord. To practice this, play the open major chord, visualize the root of that chord on the high E string, then slide into the E and G strings four frets higher. For an open C chord, we’d observe that the C note is at the 8th fret on the E string, and so we’d slide into the 12th fret (4 frets higher). This works for all major chords.

There’s a similar trick for minor chords that I’ll leave as an exercise for the reader. The key to that is that you’ll be sliding into a major third three frets above the root. Have fun with it!