In the previous article, we learned about substitute chords — swapping members within the diatonic team. Now it's time to call in reinforcements from outside the team — welcome to the world of non-diatonic chords (chords not found in the key).
⚽ Reinforcements from Outside
In the diatonic article, we met our 7 teammates — the diatonic chords. In the substitute chords article, we swapped players within that team.
But in soccer, there are loan signings and guest players, right? The team's core strategy stays the same, but at just the right moment, a specialist from outside the team steps onto the pitch.
That's exactly what non-diatonic chords are. You temporarily borrow a chord from outside the key (team). That's why they're also called "borrowed chords."
A progression made entirely of diatonic chords feels "safe." But sometimes safe isn't dramatic enough. Reinforcements bring a moment of surprise to your progressions.
🌶️ Reinforcements Are "Spice" — Don't Overdo It
Non-diatonic chords are like spice in cooking.
Too much spice in a curry and you can't taste anything. But add just a pinch of black pepper to a simple home-cooked stew, and suddenly it makes you go "oh!"
Non-diatonic chords work the same way. The trick is to mix in just 1 or 2 within a diatonic progression. Too many, and your listeners lose track of the key entirely.
A good rule of thumb: one per 4–8 bars. Start with that balance.
🎯 You Only Need to Remember 3
Non-diatonic chords come in many varieties — theory textbooks spend dozens of pages on them. But the patterns that actually show up frequently in pop and rock are limited.
In this article, we'll focus on 3 reinforcements you can start using today.
1. ♭VII (Flat Seven) — A rock staple
2. IVm (Subdominant Minor) — The tear-jerker
3. V/V (Secondary Dominant) — A chain of "pull-back" force
🔥 Reinforcement #1: ♭VII (Flat Seven) — A Rock Staple
In the key of C major, the ♭VII chord is B♭.
Normally, the 7th chord in C major is Bdim (remember from the diatonic article?). The ♭VII lowers that root by a half step to create a B♭ major chord — a chord that doesn't exist in the diatonic set.
C → B♭ → F → C
Play this progression and you'll feel a distinctly rock-flavored punch the moment B♭ enters.
Listen for it: Think of The Beatles' "Hey Jude." That famous "Na na na na…" coda uses the progression F → E♭ → B♭ → F (I → ♭VII → IV → I) in the key of F major. Transposed to C major, that's C → B♭ → F → C — exactly the progression shown above. That powerful, embracing sense of uplift? That's the ♭VII at work.
The ♭VII is borrowed from the parallel minor key. For C major, that's C natural minor — the minor key built on the same root note. In C natural minor, B♭ occurs naturally, so we're temporarily borrowing it from there.
🌧️ Reinforcement #2: IVm (Subdominant Minor) — The Tear-Jerker
In the key of C major, the IVm chord is Fm.
Normally, the IV chord (4th) in C major is F major. Change it to F minor, and the bright, floating quality dips into shadow for just a moment. This "fleeting darkness" is one of pop music's most beloved emotional turning points.
C → F → Fm → C
Play the F → Fm transition. Right after F opens up the sky, Fm lets a shadow pass across it. That contrast grabs the listener's heart.
Listen for it: Oasis's "Don't Look Back In Anger" is a textbook example of IVm. The song is in C major, and in the pre-chorus ("So I start a revolution from my bed..."), the progression F → Fm → C repeats. That bittersweet yet powerful surge of emotion? It comes from the fleeting shadow of F turning to Fm.
Note: The original recording actually uses "Fm6" — Fm with one extra note added. The fundamental role and emotional quality are the same as Fm, but the added 6th creates a deeper, lingering resonance. You can explore the Fm → Fm6 extension using OtoTheory's "Advanced" tab.
Like the ♭VII, IVm is also borrowed from the parallel minor key. Fm exists naturally in C natural minor. It's a technique of mixing just a drop of minor emotion into a major-key progression — something you'll hear constantly in movie soundtracks.
⚡ Reinforcement #3: V/V (Secondary Dominant) — A Chain of "Pull-Back" Force
This one's a bit more complex, but the payoff is huge.
In the chord progressions article, we learned that V (the striker) has a "pull-back force" toward I (the captain). So what if we created a chord that has pull-back force toward V itself?
In C major, V is G. The chord with pull-back force toward G is D (or D7). This is V/V (five of five), known as the secondary dominant. Think of it as a fake striker who delivers a powerful pass to the real striker (V). It appears from outside the team for just a moment, doubling the real striker's finishing power.
C → D → G → C
A plain C → G → C feels like "home → climax → home." But with D inserted, it becomes "home → acceleration → climax → home." It's like giving G (the striker) a running start before the big moment.
The F# in D doesn't exist in the key of C major. It's temporarily borrowed from the world of "G major" — treating G as if it were the home key (I) for just a moment. That gives the move to G the power of a V→I resolution. This technique of "briefly borrowing from another key's world" is the essence of secondary dominants.
Listen for it: Think of The Beatles' "All You Need Is Love." In the chorus, the progression is G → A → D (I → V/V → V) in the key of G major. The A chord should be Am (minor) in G major's diatonic set — but it's played as a major chord. That's the secondary dominant at work. Transposed to C major, it's C → D → G — exactly the same progression. That powerful landing on "love"? It's the "acceleration" created by V/V.
🎨 Comparing the 3 Reinforcements
| Reinforcement | Example in C Major | Effect | Listen for it |
|---|---|---|---|
| ♭VII | B♭ | Rock-flavored power | Hey Jude "Na na na na…" (The Beatles) |
| IVm | Fm | A fleeting moment of sadness | Don't Look Back In Anger (Oasis) |
| V/V | D (D7) | Doubles the pull toward the next chord | All You Need Is Love (The Beatles) |
All three come from "outside the diatonic team," but their personalities are completely different. ♭VII brings power, IVm brings sadness, and V/V brings momentum. Choose based on your goal.
We recommend starting with IVm (Subdominant Minor). Just play F → Fm → C and you'll think, "Wow, that sounds professional."
📚 The Non-Diatonic World Beyond
The 3 reinforcements in this article are just the "beginner's guide." There are many more reinforcements waiting outside the team:
| Name | In a nutshell |
|---|---|
| ♭VI (Flat Six) | A cinematic, grand sound familiar from movie scores |
| ♭III (Flat Three) | Pair with ♭VII for even more rock intensity |
| Tritone Substitution | A jazz staple — using ♭II7 in place of V7 |
| Picardy Third | Ending a minor-key piece on a major chord — a classical technique |
These will be covered in future articles. For now, master today's 3 reinforcements, then gradually expand your toolbox.
🎛️ Experience Non-Diatonic Chords in OtoTheory
In OtoTheory, when you use non-diatonic chords, they're treated not as "mistakes" but as "spice" — so you can experiment with confidence.
* Insight Engine in the Build screen: With a chord progression in place, tap any existing chord chip and the Insight Engine appears at the top of the screen. Place a non-diatonic chord, and it will automatically describe its character — for example, "Using notes outside the key — adds a spice effect." If it's a borrowed chord or secondary dominant, it shows the theoretical background too
* Insight tab: The Insight tab at the bottom of the screen gives you an overview of all sections (Verse, Chorus, Bridge, etc.). Select any chord, and it displays a list of substitute chords and borrowed chord alternatives. You can preview each one, and if you like what you hear, replace the existing chord with a single tap
* Fretboard display: View non-diatonic chord tones on the fretboard to instantly see "ah, this note is the only one outside the key." For example, placing D in C major highlights just the F# as an out-of-key note. You can visually confirm which notes are the "spice"
✅ Summary
Non-diatonic chords are reinforcements temporarily borrowed from "outside" the diatonic team. Mix just one into a diatonic progression, and you add "a moment of surprise."
* ♭VII (Flat Seven): Rock-flavored power. That soaring feeling in Hey Jude's "Na na na na…"
* IVm (Subdominant Minor): A fleeting moment of sadness. That shadow in Don't Look Back In Anger
* V/V (Secondary Dominant): Doubles the "pull-back force" toward the next chord. That surge in All You Need Is Love
* The most effective first step is to play F → Fm → C
* Don't overdo it. One per 4–8 bars is the golden ratio for spice
In the next article, we'll learn how changing just the bass note of a chord can make progressions smoother — the technique of slash chords (on-chords).