If you haven’t already, watch the video above.

It feels almost unbelievable at first: dozens of famous songs, all built from the same four chords. Different melodies, different lyrics, different artists—yet the harmony underneath barely changes.

So what’s really going on?

How can just four chords work in so many situations—and why do they sound good no matter what key they’re played in?

To understand this, we only need a small amount of music theory.

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The 12 notes of Western music

In Western music, there are 12 total notes. Each note is separated by a half-step, and together they form the chromatic scale.
After twelve notes, the pattern simply repeats at a higher pitch.

From this chromatic scale, we can build other scales by using specific step patterns.

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Major and natural minor scales

The two most common scales in popular music are the major scale and the natural minor scale.

They are defined by patterns of:

• W = whole step (two half steps)

• H = half step

Major scale pattern

• R – W – W – H – W – W – W – H

Natural minor scale pattern

• R – W – H – W – W – H – W – W

Each scale uses 7 of the 12 available notes and begins on a root note (R).

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Two simple examples

To keep things simple, here are two scales with no sharps or flats:

C major

• C – D – E – F – G – A – B – C

A minor

• A – B – C – D – E – F – G – A

For the purpose of this example, I’ve intentionally chosen these two keys.
You can apply the same scale formulas to any starting note, which will introduce sharps or flats as needed.

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Assigning Roman numerals

Next, we assign Roman numerals to each scale degree.
These numbers tell us which chord is built on each note.

Key of C major

• C = I

• D = ii

• E = iii

• F = IV

• G = V

• A = vi

• B = vii°

Key of A minor (natural minor)

• A = i

• B = ii°

• C = III

• D = iv

• E = v

• F = VI

• G = VII

Upper- and lower-case numerals indicate the chord quality (major or minor), and the ° symbol indicates a diminished chord.

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Where the “four chords” come from

Now we can finally look at the famous four chords.

In tonal music, the I, IV, and V chords are the three primary functional chords of a key.
Together, they form the basic foundation of most harmonic progressions.

These three chords account for three out of the four chords used in the video.

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Turning scale degrees into actual chords

To go from notes to chords, we simply build a chord on each scale degree.

In C major

• I → C major

• IV → F major

• V → G major

In A minor (natural minor)

• i → A minor

• iv → D minor

• v → E minor

Note: In most real music written in minor keys, the V chord is often made major (for example, E major in the key of A minor) by raising the seventh scale degree.
For simplicity, this example uses the natural minor scale.

You can think of I, IV, and V like variables in math—they change depending on the key you are in.

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What is transposing?

The process of changing these same Roman-numeral patterns into a different key is called transposing.

This is often done to better suit a singer’s vocal range—and it’s exactly what’s happening in the video.

Below is the key of C major transposed to several other keys:

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The fourth chord

So what about the final chord?

The last chord in this progression is:

• vi in a major key

• VI in a minor key

For example:

• In C major, vi is A minor

• In A minor, VI is C major

This chord is especially common in pop music because it shares two notes with the I chord.
That allows it to substitute smoothly for the tonic while still creating a noticeable emotional shift.

That contrast is one of the reasons this chord works so well alongside I, IV, and V.

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The famous progression

In the video, the chords are played in the same order throughout:

I – V – vi – IV

You can change the order of these chords, repeat them in different patterns, or add additional chords to create an almost endless number of possibilities.