A Little Chord Theory
Everything you need to know about chords — from triads to jazz voicings — explained for real humans.
1. Intervals — The Building Blocks
Before you can understand chords, you need to understand intervals — the distance between two notes. Every chord is built by stacking intervals on top of a starting note (the root).
Music divides the octave into 12 semitones (half steps). A semitone is the smallest step on a guitar — one fret. Two semitones make a whole step (also called a tone).
| Interval | Symbol | Semitones | Example (from C) |
|---|---|---|---|
| Unison | 1 | 0 | C → C |
| Minor 2nd | ♭2 | 1 | C → D♭ |
| Major 2nd | 2 | 2 | C → D |
| Minor 3rd | ♭3 | 3 | C → E♭ |
| Major 3rd | 3 | 4 | C → E |
| Perfect 4th | 4 | 5 | C → F |
| Tritone | ♭5 / #4 | 6 | C → F# / G♭ |
| Perfect 5th | 5 | 7 | C → G |
| Minor 6th | ♭6 | 8 | C → A♭ |
| Major 6th | 6 | 9 | C → A |
| Minor 7th | ♭7 | 10 | C → B♭ |
| Major 7th | 7 | 11 | C → B |
| Octave | 8 | 12 | C → C (higher) |
The key insight: The 3rd is the interval that determines whether a chord sounds major (happy) or minor (sad). The 5th provides stability. Everything else is flavour.
2. Triads — Your First Chords
A triad is the simplest chord: three notes stacked in thirds. Take any note, skip one, take the next, skip one, take the next. That gives you a root, a 3rd, and a 5th.
There are four types of triads, and they cover a huge amount of popular music:
The only difference between major and minor is the 3rd. Major has a major 3rd (4 semitones from root), minor has a minor 3rd (3 semitones). On guitar, that's literally one fret. Flatten the 3rd → you go from sunshine to rain.
3. Inversions — Same Chord, New Angle
An inversion means rearranging which note is on the bottom. The chord still has the same notes — you just change which one is the bass note (lowest).
Inversions matter because they change the voice leading — how smoothly one chord connects to the next. When you see a chord written as C/E, that means "C chord with E in the bass" (first inversion).
On guitar, you play inversions all the time without realising it. An open C chord has E as its lowest note — that's actually first inversion!
4. Seventh Chords — Adding Colour
Take a triad and add one more third on top. Now you have four notes and a lot more personality. The extra note is some flavour of a 7th.
The dominant 7th (e.g. G7) is special — it creates tension that wants to resolve to the chord a perfect 5th below (G7 → C). This "V7 → I" movement is the most powerful resolution in Western music. It's the reason the last chord before the chorus landing hits so hard.
5. Extended & Altered Chords
Keep stacking thirds beyond the 7th and you get extended chords: 9ths, 11ths, and 13ths. These are the bread and butter of jazz, R&B, and neo-soul.
Add the 9th (same as the 2nd, an octave up). C9 = C E G B♭ D. Adds brightness and sophistication without getting too weird.
The 11th (= 4th, octave up) creates a dreamy, suspended quality. Often the 3rd is omitted to avoid the clash with the 11th.
All seven notes of the scale stacked up. In practice you leave some out. The 13th (= 6th) adds a breezy, open sound. Funk and soul love these.
Altered chords take it further by sharping or flatting the extensions (♭9, #9, #11, ♭13). The infamous "Hendrix chord" (7#9) — think the opening of "Purple Haze" — is a dominant 7th with a sharp 9th. It sounds simultaneously major and minor.
9 = 2, 11 = 4, 13 = 6. They're the same notes, just an octave higher. We use the bigger numbers to signal "this is an extension on top of a 7th chord." A "Cadd9" is a C triad + the 9th (no 7th). A "C9" is a C7 + the 9th (with the ♭7 included).
6. Common Chord Progressions
Chords don't exist in isolation — they move. A chord progression is a sequence of chords, and certain sequences are so common they have names. These are written with Roman numerals referring to the scale degree of the root.
| Progression | In C major | Used in |
|---|---|---|
| I – V – vi – IV | C G Am F | Don't Stop Believin', Let It Be, No Woman No Cry |
| I – IV – V – I | C F G C | Rock & roll, country, blues (the "three-chord trick") |
| ii – V – I | Dm7 G7 Cmaj7 | Jazz standard backbone, "Autumn Leaves," countless standards |
| vi – IV – I – V | Am F C G | Despacito, Africa, Someone Like You |
| I – vi – IV – V | C Am F G | 50s doo-wop, Stand By Me, Every Breath You Take |
| I – ♭VII – IV – I | C B♭ F C | Sweet Child O' Mine, Hey Jude (verse), rock anthems |
| i – ♭VII – ♭VI – V | Am G F E | Andalusian cadence — Hit the Road Jack, Stray Cat Strut |
Roman numerals are key-independent. A "I – V – vi – IV" in G is G D Em C. In E it's E B C#m A. Same emotional shape, different key. Learn the numbers and you can play anything in any key.
7. The Nashville Number System
Session musicians in Nashville use a shorthand that takes Roman numerals even further. Instead of writing out chord names for every key, they write numbers. A chart might read:
That's a I – I – IV – IV – V – V – I – I progression. The bandleader calls out "key of A!" and everyone instantly knows: A – A – D – D – E – E – A – A. Key change to B♭? No problem — the chart stays the same.
This system is incredibly practical for worship bands, cover bands, and session work where keys are frequently changed on the fly.
8. Practice Tips
Master major, minor, and dominant 7th open chords, then barre chords. These cover 95% of popular music. Don't rush to jazz voicings before you can smoothly transition between Am, C, G, and D.
Knowing the shape of G is easy. Switching cleanly from G to C to D at tempo is the real skill. Set a metronome, pick two chords, and switch on every beat. Start slow.
In any major key, the chords built on each scale degree form a family: I(maj) – ii(min) – iii(min) – IV(maj) – V(maj) – vi(min) – vii°(dim). Learn one key thoroughly and the pattern repeats in every other key.
Theory explains why something sounds the way it does, but your ears are the final judge. Play a chord, change one note, listen to what happens. The best musicians balance knowledge with instinct.
After learning a new chord type, find a song that uses it and learn that song. Theory sticks when it's connected to real music you love.