The Chords in a Key
After this lesson, you'll know how to find the seven chords that belong to a major or minor key, how to name them with Roman numerals, and how to pick chords that go together without fumbling around.
You know how to build any chord on any note. But when you're writing, the real question is which ones go with your key. The answer is surprisingly simple. You just build a chord on each note of the scale, using only the notes of that scale. This is called harmonizing the scale.
Seven notes, seven chords
Let's take C major, the white-key scale. On each of its notes, stack two thirds while staying on the white keys, that is, every other note of the scale, as in the previous lesson. Listen to the result, from the first degree to the seventh, and then back to the first.
You heard different colors, even though you only played white keys. Some chords sound bright, others darker, and the second to last sounds downright tense. The table goes through them one by one.
| Degree | Notes | Thirds | Chord | Roman numeral |
|---|---|---|---|---|
| 1 | C E G | major, minor | C | I |
| 2 | D F A | minor, major | Dm | ii |
| 3 | E G B | minor, major | Em | iii |
| 4 | F A C | major, minor | F | IV |
| 5 | G B D | major, minor | G | V |
| 6 | A C E | minor, major | Am | vi |
| 7 | B D F | minor, minor | B° | vii° |
Roman numerals
Musicians number these chords with Roman numerals, based on the scale degree each one is built on. An uppercase numeral means a major chord, a lowercase numeral a minor chord, and the small circle, °, a diminished chord. In major, the sequence is therefore always the same: I, ii, iii, IV, V, vi, vii°.
The benefit is huge: a Roman numeral doesn't depend on the key. The progression I, V, vi, IV gives you C, G, Am and F in C major, but G, D, Em and C in G major. The numeral describes the chord's place in the scale, and so the feeling it creates, no matter what pitch you play it at. When a musician says a loop “goes one, five, six, four,” that place is what they're talking about.
Three major, three minor and one diminished
Where does this mix come from? From where the two half steps fall in the major scale, between E and F, then between B and C. When you stack thirds within the scale, a third that spans one of those half steps covers only three half steps and becomes minor; a third that spans neither covers four and stays major. From C to E, there's no half step along the way: the third is major. From D to F, you cross the half step between E and F: the third is minor.
The result is three major chords, on degrees 1, 4 and 5, three minor chords, on degrees 2, 3 and 6, and one diminished chord on degree 7, the only one whose two thirds are both minor. That last one contains the tritone between B and F, which explains its tension. It's fairly rare on its own in electronic music; you mostly come across it as a passing chord, or with a seventh added.
The chords of a minor key
Let's do the same thing with A minor, which shares the notes of C major. So the chords are the same, but they get new numbers, because home is no longer in the same place: the first degree is now A.
In natural minor, the sequence of qualities becomes minor, diminished, major, minor, minor, major, major, which is written i, ii°, III, iv, v, VI, VII. It's the same sequence as in major, but it starts on the sixth chord, since A is the sixth degree of C major.
| Degree | Chord in A minor | Roman numeral |
|---|---|---|
| 1 | Am | i |
| 2 | B° | ii° |
| 3 | C | III |
| 4 | Dm | iv |
| 5 | Em | v |
| 6 | F | VI |
| 7 | G | VII |
Here's an important point. In many songs in minor, the chord on the fifth degree isn't minor but major: you replace G with G♯, and Em becomes E. That G♯ comes from the harmonic minor scale, which you already know: it creates a very strong pull toward A. You then write V instead of v, and the rest of the key doesn't change. Listen to the difference.
You can hear it: after Em, the return to Am is gentle; after E, it feels almost inevitable. We'll see why in the lesson on tension and resolution.
Finding chords that go together
Here's the practical consequence. As long as you stick to the seven chords of your key, they all go together, because they're made of the same seven notes. A melody written in the scale will work with each of them, with more or less rubbing depending on the note. You're no longer choosing at random among dozens of chords: you're choosing among seven.
Among those seven, some are closer than others. Two chords that sit next to each other in the list of degrees share no notes, while two chords a third apart share two. C and Am, for example, have C and E in common; F and Dm share F and A. These close chords easily stand in for each other. If you don't like a chord in a loop, try the one a third higher or lower: the loop keeps its meaning and changes color.
Now let's test the idea behind Roman numerals. Listen to the loop, then change the key without touching the numerals.
In C major, your progression gives C, Em, Am, F, one chord per bar. Click a chord in your progression to select it, then a roman numeral to replace it: the numerals stay the same in every key, only the chord names change.
Borrowed chords come from another scale on the same tonic: ♭VI, ♭VII and iv come from minor and darken the loop.
Harmonize the G major scale, which contains a single sharp, F♯. Write down the seven chords with their Roman numerals, then play the progression vi, IV, I, V in your software. Then do the same for D minor, whose scale contains one flat, B♭.
So far, we've always played chords the same way, with the root at the bottom. The next lesson teaches you to flip them and spread them across the keyboard so they connect smoothly.