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The problem it solves
Real progressions do not grab chords at random: almost all of them come from a single key. Knowing which seven chords belong to a major scale and what character each one has is the first step to figuring out songs, composing and analysing harmony.
Detailed theory
Key idea
Each degree of the major scale generates a triad made only of notes of the scale: seven chords in total.
The qualities always follow the same pattern: I IV V major, ii iii vi minor and vii° diminished.
Understand it
Start from the major scale and, on each note, stack two thirds taking only notes of that same scale (the note, skipping one, and skipping another). Since every note comes from the scale, the resulting chords belong to it: that is why they are called diatonic.
In C major the seven chords are C (I), Dm (ii), Em (iii), F (IV), G (V), Am (vi) and B° (vii°). Notice that you have used no accidentals: it is all white keys spread across seven triads.
The qualities are not random; they depend on where the tones and semitones of the scale fall. The result is always the same pattern: I, IV and V are major; ii, iii and vi are minor; and vii°, built on the leading tone, is diminished. In Roman numerals this is written with uppercase for the major chords and lowercase for the minor ones: I ii iii IV V vi vii°.
I, IV and V are the three major chords, called primary chords because they hold up the key (tonic, subdominant and dominant). With just these three you can already harmonise a huge number of songs.
An analogy: the major scale is a family, and each degree is a sibling built from the same material but taken from a different spot of the scale. Seven chords that share the same base notes and yet each has its own character within the home.
Chord progression
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The three primary chords of C major: I (C), IV (F) and V (G). All three are major and hold up the key.
How to recognise it
How it's written
They are written with Roman numerals according to the degree: uppercase for the major chords (I, IV, V), lowercase for the minor ones (ii, iii, vi) and the symbol ° for the diminished (vii°). This notation is independent of the key: the pattern I ii iii IV V vi vii° works for any major scale.
How it feels
Play the seven chords in a row, from I to vii°: they sound like a coherent family. The I sounds like home, the V asks to return to it and the vii° is the most unstable. It does not mean the major chords are happy and the minor ones sad; it is more useful to speak of brightness, rest and tension.
Common mistake
Thinking the chords of a key can be of any quality: the major-minor-diminished pattern is fixed in every major key.
Forgetting the vii° and treating it as a minor chord: built on the leading tone, it is diminished (it has a lowered fifth).
Try it
Play the seven triads of C major one after another (C, Dm, Em, F, G, Am, B°) and say out loud the quality of each one.
Chain only the primary chords I-IV-V-I in C (C-F-G-C) and notice how the three major chords already give a complete, resolved progression.
On the instrument
Chord progression
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The seven diatonic triads of C major: I (C) ii (Dm) iii (Em) IV (F) V (G) vi (Am) vii° (B°). No accidentals: all notes of the scale.
Diatonic chords
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The notes of the C major scale show up faint; tap each diatonic chord to see which scale notes it lights up (I C, ii Dm, iii Em, IV F, V G, vi Am, vii° B°).
Where it's used
- Figuring out songs by ear
- Recognising that most chords of a song come from a single key and deducing them from the seven diatonic degrees.
- Composing coherent progressions
- Choosing chords from the same key (for example I-vi-IV-V) so they sound meaningfully together.
- Analysing with Roman numerals
- Labelling the chords of a progression by their degree (I, ii, V…) to understand their function in any key.
Examples
Chord progression
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A I-IV-V-I progression in C major: it leaves the tonic, passes through subdominant and dominant and returns home. Only with diatonic chords.
Exercises
Recognise the diatonic chords — basic
Tell apart the major and minor triads that appear in a major key.
Complete 5 attempts · 70% accuracy to pass
Recognise the diatonic chords — intermediate
Add the diminished triad (degree vii°) to the mix of qualities.
Complete 8 attempts · 70% accuracy to pass
Recognise the diatonic chords — advanced
Master the three triad qualities found in a major key.
Complete 10 attempts · 70% accuracy to pass
Mini test
Check that you've got it.
0/10 answeredQuestion 1/10
What does it mean for a chord to be diatonic to a key?