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

Know What Chords Are

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!

George Markus

George Markus is our music theory guru who works in social media. He’s been playing guitar for 20-years and holds a master’s degree in composition and, you guessed it, music theory. When he’s not jamming out to rock and blues songs, he plays classical music for guitar.
Now, he’s using his knowledge to answer your questions in his new weekly editorial George’s Corner.