Diatonic Chords and Their Functions

Welcome to our next lesson on the fundamentals of music theory. In our last session, you learned a systematic method for building the three primary types of triads—major, minor, and diminished—from any starting note. This was about constructing the individual "words" of harmony.

Today, we will learn how to form "sentences" by understanding which chords naturally belong together within a given musical key. Our goal is to identify the diatonic chords (the family of chords built from a single scale) for any major or minor key and to understand their specific roles, or functions. This is a crucial step toward composing the kind of compelling, emotionally resonant chord progressions that are a hallmark of the deadmau5 sound.

The Diatonic System: Building Chords Within a Key

The term "diatonic" simply means "using the notes of the scale." To find the diatonic chords of a key, we apply the triad-building process you've already learned, but with one constraint: we can only use notes available in that key's scale.

Let's use the C major scale as our model: C, D, E, F, G, A, B.

We can build a triad starting on each of these seven notes (also called scale degrees). The process is always to take a root note, skip a scale note, take the next (the third), skip another, and take the next (the fifth).

1. Chord on C: Root C, skip D, take E, skip F, take G -> C-E-G. Let's analyze this using the semitone formulas from last lesson. From C, E is +4 semitones and G is +7 semitones. This is a C Major triad.
2. Chord on D: Root D, skip E, take F, skip G, take A -> D-F-A. From D, F is +3 semitones and A is +7 semitones. This is a D minor triad.
3. Chord on E: Root E, skip F, take G, skip A, take B -> E-G-B. From E, G is +3 semitones and B is +7 semitones. This is an E minor triad.

If we continue this process for all seven scale degrees, we get the following set of chords:

• C-E-G -> C Major
• D-F-A -> D minor
• E-G-B -> E minor
• F-A-C -> F Major
• G-B-D -> G Major
• A-C-E -> A minor
• B-D-F -> B diminished (B to D is +3st, B to F is +6st)

Roman Numeral Notation and Chord Functions

Notice the pattern of chord qualities that emerged: Major, minor, minor, Major, Major, minor, diminished. This pattern is the same for every major key. To describe this pattern abstractly, without reference to a specific key like C major, we use Roman numerals.

• Uppercase numerals denote Major chords.
• Lowercase numerals denote minor chords.
• A lowercase numeral with a degree symbol (°) denotes a diminished chord.

So, the diatonic triads of any major key are: I, ii, iii, IV, V, vi, vii°.

These chords don't just exist as a random collection; they serve distinct roles in creating a sense of movement, tension, and resolution. This concept is called functional harmony. The following video provides an excellent breakdown of this system.

To summarize the video's key points, the seven diatonic chords are grouped by function:

• Tonic (Stability/Home): I, iii, vi. These chords create a sense of rest or resolution. The I chord is the primary tonic, the ultimate 'home base' of the key.
• Subdominant (Transition/Movement): IV, ii. These chords create moderate tension and are often used to lead away from the tonic and towards the dominant.
• Dominant (Tension/Resolution): V, vii°. These chords create the strongest tension and have an intense "pull" back to the tonic. The V chord is the primary dominant.

A typical harmonic path follows this logic: Tonic -> Subdominant -> Dominant -> Tonic.

Harmony in Minor Keys: The Progressive House Palette

While understanding major key harmony is fundamental, progressive house is predominantly written in minor keys. This is where the harmonic language becomes richer and more flexible. The process is the same—build triads from a scale—but we have more than one minor scale to draw from.

We begin with the natural minor scale. Let's use A minor, which has no sharps or flats (A, B, C, D, E, F, G). The diatonic chords are:

• i: A minor (A-C-E)
• ii°: B diminished (B-D-F)
• III: C Major (C-E-G)
• iv: D minor (D-F-A)
• v: E minor (E-G-B)
• VI: F Major (F-A-C)
• VII: G Major (G-B-D)

The pattern is: minor, diminished, Major, minor, minor, Major, Major.

The following video is an excellent deep dive into minor key harmony. It will demonstrate how these chords are derived and, crucially, introduce the variations that give minor key music its expressive power.

The Power of the Minor Key Palette

As you saw in the video, the "problem" with the natural minor scale is that its v chord is minor. This chord lacks a leading tone—a note that is just one semitone below the tonic and which creates a powerful magnetic pull back to it. In A minor, the notes of the v chord (E minor) are E, G, and B. The G natural is a whole step away from the tonic A, creating a less compelling resolution.

To fix this, composers frequently "borrow" the raised 7th degree from the harmonic minor scale. In A minor, this means replacing G with G♯. This one change has a significant effect:

• The v chord (E minor: E-G-B) becomes a V chord (E Major: E-G♯-B). This chord now contains the G♯ leading tone, creating a much stronger V -> i resolution.
• Other chords containing the 7th degree also change. The VII chord (G Major) becomes a vii° chord (G♯ diminished).

This practice of mixing chords from the natural and harmonic minor scales is standard. It gives you the best of both worlds: the smooth, dark quality of the natural minor chords (like the III, VI, and VII) and the powerful, driving resolution of the harmonic minor's V chord. This is the system that generates iconic progressive house progressions like i - VI - III - VII (using all natural minor chords) and i - VI - III - V (using the borrowed dominant for a stronger finish).

Practical Summary: The Diatonic Chord Cheat Sheet

Here is a consolidated view of the diatonic triads you'll use most often.

Major Key (e.g., C Major):

Numeral	Chord in C	Quality	Function
I	C Major	Major	Tonic
ii	D minor	minor	Subdominant
iii	E minor	minor	Tonic
IV	F Major	Major	Subdominant
V	G Major	Major	Dominant
vi	A minor	minor	Tonic
vii°	B dim	diminished	Dominant

Minor Key (e.g., A Minor):
(This table shows the most common chords, combining natural minor with the borrowed V from harmonic minor).

Numeral	Chord in A	Quality	Source	Function
i	A minor	minor	Natural/Harmonic	Tonic
ii°	B dim	diminished	Natural/Harmonic	Subdominant
III	C Major	Major	Natural	Tonic
iv	D minor	minor	Natural/Harmonic	Subdominant
V	E Major	Major	Harmonic	Dominant
v	E minor	minor	Natural	Dominant
VI	F Major	Major	Natural/Harmonic	Tonic
VII	G Major	Major	Natural	Dominant/Subdom

The most important takeaway for our purposes is the interchangeability of v and V in a minor key. You can use the minor v for a softer, more ambiguous feeling, or the Major V for a powerful, definitive resolution.

Conclusion

Today, we've unlocked the system that governs harmony within a key. You can now not only build individual chords, but you can also generate the entire family of seven diatonic chords for any major or minor key and understand their fundamental roles.

Here are the key takeaways:

• Diatonic chords are the triads built using only the notes of a specific scale.
• Roman numeral notation (I, ii, iii, etc.) provides a key-independent way to analyze chord patterns.
• Chords have functions (Tonic, Subdominant, Dominant) that create a narrative of tension and release.
• Minor keys offer a richer harmonic palette by borrowing chords from the natural, harmonic, and melodic minor scales, most notably using the Major V chord from the harmonic minor to create a strong resolution.

You now have the complete theoretical framework for understanding chords within a key. In our next lesson, we will put this knowledge into direct practice by constructing some of the most common and effective four-chord progressions used in progressive house music.