Functional Harmony

I picked up most of this functional harmony stuff through jamming with friends in the back of David's Guitar Loft. Aaron would usually lead the jams and we would play over songs like Down with Disease by Phish, Holiday in Cambodia by Dead Kennedys, I Wanna Be Sedated by the Ramones, Holiday by Green Day, Purple Haze by Hendrix, and lots of classic rock. Sometimes it became a game of how I could reharmonize a song but still keep within the bounds of it. I would sub chords out. A lot of the time it would work, a lot of the time I'm sure it didn't.

I learned the "real rules" of Western harmony much later when I went to music school briefly to get a minor. At that point I started to understand these fundamental relationships and how they were rooted in Western music history, like different cadences and how they developed over time from Gregorian chants to Bach and then to the blues and jazz. These music rules can be extremely helpful and interesting, but at the end of the day, always go with your ear and what it's telling you to play. I hope this is more informative and easier to digest than a textbook. Cheers!

Note: this post is written in the context of Western music theory. Other musical traditions have their own harmonic systems that don't necessarily follow these rules. Only a Sith speaks in absolutes.

Every major key is really just 3 chords

Chords in a major key typically function as one of three things: tonic, subdominant, or dominant.

The three functions

Tonic (home)

Where tension resolves. In the key of C:

• C (I)
• Am (vi), shares C and E with C

Subdominant (departure)

Moving away from home. In the key of C:

• F (IV)
• Dm (ii), shares D and F with F, often used in place of F especially in jazz (ii-V-I)

Dominant (tension)

Wants to go home. In the key of C:

• G (V), can be made even stronger with altered extensions like #9 or b9
• Bm7b5 (vii), shares B and D with G7. G and Bm7b5 are functionally interchangeable. Bm7b5 is essentially a G9 without the root.
• Em (iii), shares B and G with G. I would call it a soft dominant. It can be useful for playing Em pentatonic licks over a G turnaround in C.

Why this works

Chords that share two out of three notes can fill the same role:

C:   C  E  G
Am:  A  C  E   ← shares C and E with C

F:   F  A  C
Dm:  D  F  A   ← shares F and A with F

G:   G  B  D
Bm7b5: B  D  F  ← shares B and D with G
Em:  E  G  B   ← shares G and B with G

The simplest way to think about it

Function	Primary chord	Substitutes
Tonic	C (I)	Am (vi)
Subdominant	F (IV)	Dm (ii)
Dominant	G (V)	Bm7b5 (vii), Em (iii)

So when you see a progression like:

C → Am → Dm → G → C

You're really hearing:

Tonic → Tonic → Subdominant → Dominant → Tonic

And a progression like:

C → Am → F → Dm → Em → G → C

Is still just:

Tonic → Tonic → Subdominant → Subdominant → Dominant → Dominant → Tonic

Try it yourself

Pick any song in a major key. Label each chord as tonic, subdominant, or dominant.

• Reharmonize songs by swapping chords within the same function
• Predict what chord comes next
• Transpose by thinking in functions instead of letter names

Pentatonic scales and functional harmony

Because chords in the same function share notes, their pentatonic scales overlap too.

Tonic pentatonics

C major pentatonic and A minor pentatonic are the same notes:

C major pentatonic: C  D  E  G  A
A minor pentatonic: A  C  D  E  G

Over any tonic chord (C or Am), either pentatonic works.

Subdominant pentatonics

F major pentatonic and D minor pentatonic are the same notes:

F major pentatonic: F  G  A  C  D
D minor pentatonic: D  F  G  A  C

Over F or Dm, either one works. These share 3 notes with the tonic pentatonics (C, G, A), which is why you can get away with staying in one pentatonic scale for most of a song.

Dominant pentatonics

G major pentatonic and E minor pentatonic are the same notes:

G major pentatonic: G  A  B  D  E
E minor pentatonic: E  G  A  B  D

Over G, Bm7b5, or Em, either one works.

Why this matters

Shift your pentatonic scale to match the function:

C       | F       | G       | C
C pent  | F pent  | G pent  | C pent
Am pent | Dm pent | Em pent | Am pent
tonic   | subdom  | dom     | tonic

Or just stay on C major pentatonic the whole time. Shifting pentatonics gives you more color and makes the changes audible in your solo.

Blues scale and functional harmony

The blues scale is a minor pentatonic with an added b5. C minor blues:

C minor blues: C  Eb  F  Gb  G  Bb

This works over a C major chord even though it has a minor third (Eb) and a flat seventh (Bb). That clash between the minor third in the scale and the major third in the chord is the blues sound.

You can shift the blues scale the same way:

C       | F       | G       | C
C blues | F blues | G blues | C blues
tonic   | subdom  | dom     | tonic

F minor blues: F  Ab  Bb  B  C  Eb
G minor blues: G  Bb  C  Db  D  F

Or just stay on C minor blues over everything. The b3, b5, and b7 act as tensions that resolve to chord tones. The Eb bends up into E (the major third of C), the Gb slides into G (the fifth).

Mixing major pentatonic with the blues scale gives you both the major third (E) and the minor third (Eb). Alternating between them is a common cliche in blues and rock soloing.

Bluegrass

Bluegrass is built on I-IV-V. Most fiddle tunes and flatpicking solos live in the major pentatonic with one important addition: the b7.

In the key of G (the most common bluegrass key):

G major pentatonic: G  A  B  D  E
G mixolydian:       G  A  B  C  D  E  F

That F natural (the b7) shows up everywhere. Fiddle tunes, banjo rolls, flatpicking runs. It gives the music that mixolydian flavor without going full blues. The b7 is usually a passing tone or a quick hammer-on, not a note you sit on the way you would in blues. I like it especially on beats 3 and 4 for adding a bit of tension before getting to your next chord.

Shifting pentatonics works the same way:

G       | C       | D       | G
G pent  | C pent  | D pent  | G pent
tonic   | subdom  | dom     | tonic

Chromatic approach notes are a big part of the bluegrass sound too. Hammering on from a half step below into a chord tone (like F# into G, or C# into D) does the same job as a blues bend but picked.

Blues leans into the minor pentatonic and bends up into major chord tones. Bluegrass stays on the major pentatonic side and uses the b7 and chromatic approaches as seasoning. Both are working with the same I-IV-V functional harmony underneath.

Plagal cadence (turnaround)

IV→I, subdominant straight to tonic. No dominant. This is the "amen" cadence you hear at the end of hymns and all over gospel music.

In C:

F → C

The I-V-vi-IV progression

The most common chord progression in pop music:

C → G → Am → F

In functional terms:

Tonic → Dominant → Tonic → Subdominant

Play these chords and I am sure you will hear a familiar song.

Another common pop progression:

I-vi-IV-V

C | Am | F | G

Rhythm Changes I-vi-ii-V

This is all over jazz tunes and creates a strong resolution from the V to I. It also navigates in fifths, providing a greater feeling of resolution to each chord.

C | Am | Dm | G

Tonic → Tonic → Subdominant → Dominant

Circle of fifths movement

Chords naturally want to move in fifths. In the key of C:

Em → Am → Dm → G → C
iii → vi → ii → V → I

In functional terms:

Dominant → Tonic → Subdominant → Dominant → Tonic

Each chord's root is a fifth above the next. The ii→V→I part is the backbone of jazz. Extending it backwards through vi and iii gives you a longer runway.

Deceptive cadence

V resolves to vi instead of I:

G → Am   (instead of G → C)

This works because Am is a tonic substitute. The dominant resolves, but lands on the "other" tonic. Am shares two notes with C (C and E), so it sounds surprising but not wrong. It's a way to extend a phrase or delay the final resolution.

Borrowed chords

Borrowed chords come from the parallel minor key. If you're in C major, you can borrow chords from C minor.

C major:

C  Dm  Em  F  G  Am  Bdim

C minor:

Cm  Ddim  Eb  Fm  Gm  Ab  Bb

The most common borrowed chords in C major:

• Bb (bVII), a major chord one whole step below C. "Fire on the Mountain" by the Grateful Dead is a great example.
• Ab (bVI), often paired with bVII. "All Along the Watchtower" uses a I→bVII→bVI→bVII movement.
• Fm (iv), the minor version of IV. Swapping F for Fm adds a darker subdominant color. "Waste" by Phish does this: D→GMaj→Gm→D, the IV goes major then minor before resolving back to I.
• Eb (bIII), a major chord built on the minor third.

These borrowed chords mostly serve a subdominant function. They come from the parallel minor, which shares the same root note.

Borrowing works in minor keys too. "House of the Rising Sun" by the Animals is in A minor:

Am   C        D           F
There is a house in New Orleans
Am        C      E     E
They call the "Rising Sun"

The D major (IV) is borrowed. In A natural minor, the iv chord is Dm. The D major brings in F# from A Dorian. The E major (V) is also not in A natural minor, where you'd get Em (v). The E major comes from A harmonic minor, raising G to G# to create a leading tone that pulls back to Am. Using a major V chord in a minor key is so common that most people don't think of it as borrowing, but it is.

Advanced

Everything below gets into music school territory, but is also useful if you want to understand why certain chords sound the way they do.

Diminished and augmented chords

Diminished and augmented chords both serve a dominant function. Diminished chords are built by stacking minor thirds, augmented chords are built by stacking major thirds.

Diminished

We already have Bm7b5 (vii) as a dominant substitute. A fully diminished 7th chord takes it further. In the key of C:

Bdim7: B  D  F  Ab
G7b9:  G  B  D  F  Ab

Bdim7 is a G7(b9) without the root. Same notes, same pull toward C.

Diminished 7th chords are symmetric, built entirely in minor thirds. Bdim7, Ddim7, Fdim7, and Abdim7 are all the same chord. This lets a single diminished shape resolve to four different keys.

Diminished chords also work as chromatic passing chords. In C:

C → C#dim7 → Dm
Dm → D#dim7 → Em
F  → F#dim7 → G

Each passing diminished chord is functioning as a rootless dominant 7(b9) of the chord it resolves to.

Augmented

The augmented triad has a raised 5th that creates a leading tone pulling up by half step. In the key of C:

G+:  G  B  D#

The D# wants to resolve up to E, which lands you on C (C E G) or Am (A C E). G+ is a more tense version of a regular G dominant chord.

G+ → C
G+ → Am

Like diminished chords, augmented triads are symmetric (built in major thirds). G+, B+, and D#+ are all the same chord. C+ (C E G#) can resolve to F, functioning as a dominant of the subdominant:

C+ → F

The G# in C+ pulls up to A, the third of F.

Symmetric scales

Diminished and augmented chords are symmetric, and the scales built from them divide the chromatic scale into equal parts.

Diminished: dividing the chromatic scale into 4

There are only 3 diminished 7th chords. Each one contains 4 notes spaced in minor thirds (3 half steps apart):

Cdim7:  C  Eb  Gb  A
C#dim7: C#  E  G   Bb
Ddim7:  D   F  Ab  B

Every other diminished 7th chord is just a respelling of one of these three.

The diminished scale (half-whole or whole-half) alternates between half steps and whole steps, giving you an 8-note scale:

C half-whole diminished: C  Db  Eb  E  Gb  G  A  Bb
C whole-half diminished: C  D   Eb  F  Gb  Ab  A  B

Half-whole works over dominant chords (play it over G7, starting on G). Whole-half works over diminished chords. Because the pattern repeats every minor third, the same diminished scale works over 4 different root notes.

Augmented: dividing the chromatic scale into 3

There are only 4 augmented triads. Each one contains 3 notes spaced in major thirds (4 half steps apart):

C+:  C   E   G#
Db+: Db  F   A
D+:  D   F#  Bb
Eb+: Eb  G   B

Every other augmented triad is a respelling of one of these four.

The augmented scale alternates between minor thirds and half steps:

C augmented scale: C  D#  E  G  G#  B

This scale contains two augmented triads a half step apart (C+ and E+). "Giant Steps" is built around augmented triad relationships, dividing the octave into three equal key centers (B, G, Eb).

Why symmetry matters

The practical benefit of these symmetric shapes is getting a more unique color on the dominant sound. Because the intervals repeat evenly, the shapes are easy to memorize on guitar or other highly symmetrical instruments. One diminished shape works in four keys, one augmented shape works in three. Learn a few shapes and you have access to a lot of dominant tension without much extra memorization.

Conclusion

This is the foundation western harmony builds on. In the next post, we'll discuss secondary dominants, dominant chords that resolve to keys outside of your home key, letting you travel even further from home.

Playing Changes