How to Know What Chords Are In What Key, And Why

I have received many questions like: “Why are certain chords used in one key?” and “Why do those chords consists of certain notes?” So, in this article, I decided I would explain a simple fact that answers all of these questions. That’s the principle of transformation from the horizontal dimension of a key (called a scale) into a vertical dimension known as a chord!

Know What Chords Are…

So, each chord (triad) is the simultaneous sounding of three notes of a scale, and could be presented as a root note (by which the chord is named) along with its 3rd and with the 3rd of its 3rd (or the 5th of the root) note.

Or, in other words:

I chord/triad consists of 1st note +3rd note +5th note of the scale; e.g. in the C major key it would be C+E+G=C major chord.

George's Corner Chords 1

ii chord/triad consists of 2nd note +4th note +6th note of the scale; so, D+F+A=D minor chord.

George's Corner Chords 2

iii chord/triad consists of 3rd note +5th note+7th note of the scale; so, E+G+B=E minor chord.

George's Corner Chords 3

IV chord/triad consists of 4th note +6th note +8th note of the scale – which is actually the 1st note again but in next or higher octave; so, F+A+C=F major chord.

George's Corner Chords 4

V chord/triad consists of 5th note +7th note +2nd note of the scale (because after the 7th note all notes repeat in the next or higher octave); so, G+B+D=G major chord.

George's Corner Chords 5

vi chord/triad consists of 6th note +1st note +3rd note of the scale; so, A+C+E=A minor chord.

George's Corner Chords 6

vii chord/triad consists of 7th note +2nd note +4th note of the scale; so, B+D+F=B diminished chord.

George's Corner Chords 7

So, that’s how we get basic chords called triads, and if we look at those chords we see that major chords are on I, IV, and V notes, and minor are on ii, iii, vi (& diminished on vii), because the major scale has the structure W-W-H-W-W-W-H, which determines whether a chord will be major or minor, and etc.

If we add one more note, on the same principle of each second note of the scale, we will get 7th-chords (because the root and top notes are the 7th)!

I chord would be 1st+3rd+5th+7th note; so C+E+G+B= C Maj7.

George's Corner Chords 8

II chord would be 2nd +4th +6th +1st note; so D+F+A+C=Dm7.

George's Corner Chords 9

V chord would be 5th +7th +2nd +4th note; so G+B+D+F=G7 (known as Dominant 7th chord), etc.

George's Corner Chords 10

The same way we make more extended chords as 9th-, 11th-, 13th-chords … just layering third on third!
– e.g. Dominant ninth chord has 5 notes:

George's Corner Chords 11

And finally, we come to the meaning of the term key, which has nearly the same meaning as scale. Think of it this way – key means a group of pitches, which in the vertical sense are formed from a scale and in a horizontal sense form chords.

This is exactly the same for any other tonality because the ratio of notes in a scale is constant W-W-H-W-W-W-H, and here is a video where Steve shows you how each key has the same I, IV, V and ii, iii, vi chords:

If you want to deepen your understanding of music, and what chords are, GuitarZoom offers excellent courses on music theory: Music Theory Made Easy, Songfire, and Just Enough Theory. Also, check out last week’s guide to Modes for Guitar!