What a chord is

A chord is what you get when you play more than one note at the same time — two notes, three notes, four, more — and they sound coherent together.

That’s the loose definition. The useful definition is more specific.

In Western music, chords are built by a single rule that’s almost embarrassingly simple:

Stack notes that are a third apart from a scale.

That’s it. Once that rule clicks, every chord in every key in every song you will ever play becomes derivable rather than memorized.

Building a chord from a scale

Take C Do major. The seven notes are:

C – D – E – F – G – A – B

To build a chord starting on C Do :

1. Start on C Do .
2. Skip the next note ( D Re ).
3. Take the one after ( E Mi ).
4. Skip again ( F Fa ).
5. Take the next ( G Sol ).

The three notes — C Do , E Mi , G Sol — played together are the C major chord:

1. C
2. C#
3. D
4. D#
5. E
6. F
7. F#
8. G
9. G#
10. A
11. A#
12. B

The notes in the chord are highlighted; the root ( C Do ) is in rust.

Why “thirds”?

The interval from C Do to E Mi is a third (it spans three letter names — C, D, E). The interval from E Mi to G Sol is also a third (E, F, G).

The skip-a-note rule and the “stack thirds” rule are the same rule. Every time you skip a scale degree, you’re moving up by an interval of a third.

Building a chord on any note

The same rule works starting from any note of the scale. Build a chord on F Fa : start on F Fa , skip G Sol , take A La , skip B Si , take C Do .

F – A – C

That’s the F major chord — and like every chord, its root is the note you started from. The chord takes its name from its root.

Try one more — build a chord on A La : start on A La , skip B Si , take C Do , skip D Re , take E Mi .

A – C – E

That’s the A minor chord. Same rule, same scale ( C Do major), different starting note, different chord quality.

Why is one chord “major” and the next “minor”?

You used the same procedure for both chords, but C-E-G sounds bright and A-C-E sounds dark. That’s not magic — it’s about the specific intervals between the stacked notes:

• C major: C to E is a major third (4 half steps). E to G is a minor third (3 half steps).
• A minor: A to C is a minor third (3 half steps). C to E is a major third (4 half steps).

The order of the two thirds is reversed, and that single difference produces two completely different emotional colors. Major thirds on the bottom sound bright. Minor thirds on the bottom sound dark.

The next chapter shows that every degree of the major scale produces a specific chord quality, in a fixed pattern that explains the Roman numeral case rule from Part II.

Recap

• A chord is two or more notes played together — but the useful kind are built by stacking thirds from a scale.
• A three-note chord is called a triad.
• A chord’s name comes from its root — the note you started on.
• The chord’s quality (major, minor, etc.) is determined by which two thirds — major or minor — you ended up stacking.