A chord is defined as any combination of two or more intervals. The most basic and fundamental form of chord is the Triad. Triads are called triads not because they have three notes, but because they are built on combinations of 3rd Intervals.

There are 4 types of triad, before we go into that, we need to understand that notes that are intervals of a root also have intervals between the other intervals of that root. A Perfect 5th is a Minor 3rd above a Major 3rd from the root.

The four triads are:

Major TriadRoot + Major 3rd + Minor 3rd (Perfect 5th)
Minor TriadRoot + Minor 3rd + Major 3rd (Perfect 5th)
Diminished TriadRoot + Minor 3rd + Minor 3rd (Diminished 5th)
Augmented TriadRoot + Major 3rd + Major 3rd (Augmented 5th)

These are the four possible ways to “stack” thirds, by playing the root, a third, and a third from that third. Your basic Major / Minor chords contain only these 3 intervals and their octaves. For example, the basic open E Major chord on the guitar, from lowest note to highest note, is Root, Perfect 5th, Octave, Major 3rd, Perfect 5th, Octave.

Let’s look at how these triads fit onto a fretboard, I’ll include the closest 3rds and 5ths to the root to show you multiple ways to approach the chord, but remember we just need 1, 3, and 5 for this triad.

In Major and Minor, these thirds alternate, and we can keep stacking alternating thirds to include the 7th, the 9th, the 11th and so on. You’ll notice that, similar to the inversion principle described earlier, the 9th is a 2nd from the next octave, the 11th is a 4th etc. If we stack 7 alternating thirds we will eventually include the entire 7 note diatonic scale for the mode we’re in.

The dominant / minor 7th is the most common 7th chord (notated as 7), but the diatonic extension of the Major Triad would be the Major 7th chord (notated as Maj7).

You can stack other intervals to form chords, but you may find what you’re playing is actually an inversion of one of these basic triads. For example, a chord comprised of: Root + Minor 3rd + Perfect 4th (Minor 6th from root) is actually just the first inversion of the triad that begins on that Minor 6th.

Because of how humans hear harmony and dissonance, a combination of root + 4th will more often be heard as that 4th plus it’s 5th. The most stable frequency division is what will be heard, something to keep in mind as you learn to construct new chord voicings.

Practice

Take some time to analyze some of the scales you may already know, and consider the interval values between the non-root intervals. What is a Minor 6th in relation to a Perfect 5th?

Can you identify any new voicings for some basic chords you already know given this information?

Can you extend some of the chordal playing your used to to include the 7th, Maj7th, 9th or 11th intervals?