A key is not just a note to start on. It is a set of seven notes, and if you build a chord on each of those notes using only the others, you get seven chords that belong together. Nearly every song you know lives inside one of these sets, which is why a song can be four chords long and still sound finished.
Where the chords come from
Take the scale and stack thirds: from each note, skip one, take the next, skip one, take the next. In C major that gives C E G on the first degree, which is a major chord, and D F A on the second, which is a minor one. Nobody decided that. It falls out of the scale, and because every major scale has the same gaps in the same order, every major key produces the same pattern.
| Degree | Chord | In C | What it does |
|---|---|---|---|
1 | I | C | home |
2- | ii | Dm | leans toward the five |
3- | iii | Em | the softer home |
4 | IV | F | away from home, and warm about it |
5 | V | G | the pull back |
6- | vi | Am | the relative minor, home with the light off |
7° | vii° | Bdim | unstable, and rarely used on its own |
Three major, three minor, one diminished, always in that order. Learn it once and you have it in all twelve keys, because the numbers do not move when the key does. That is the same reason the charts in this app are written in numbers.
Three of them, in one place
The family is not an abstraction: it is chords you can reach. These three are the 1, the 4 and the 6- of C, all found without moving your hand far, and between them they play a very large number of songs.
The minor family
A minor key uses the same seven notes as the major key three semitones above it, so it produces the same seven chords. What changes is which one you call home. Start on A rather than C and the family reorders itself around it.
| Degree | Chord | In A minor | What it does |
|---|---|---|---|
1- | i | Am | home |
2° | ii° | Bdim | unstable, and rarely used on its own |
3 | III | C | the relative major, home with the light on |
4- | iv | Dm | away from home |
5- | v | Em | the pull back, minor unless you raise it |
6 | VI | F | the warm one |
7 | VII | G | the walk down from home, and the way back up |
What it is for
Two things. When you are working a song out by ear, the family tells you what to try: if a song is in G, the chord you cannot name is far more likely to be one of the other six than something from outside. And when you already know the chart, the degrees the song leaves alone tell you as much as the ones it uses.
A song that lives on 1, 4 and 5 is bright, because those are the three major ones. A song that leans on 6- and 2- is not, and it is not the melody doing that, it is the family.