The relationship between the key of a song and the chords that are used in the song follows some very basic guidelines. This article explains how it’s all put together.
In my last article, Guitar Chord Theory – Hang-in There Its a Long Story we looked at scales and the order of steps and half-steps that are used to make a scale. If you haven’t read it you probably should as it will make this article much easier to follow. It’s important to remember that different scales have a different number of sharps or flats in them.
A key for our purpose is a bunch of related chords that sound good together. If you already play guitar you may have noticed that a song that starts with a C chords is very likely to have F & G chords with it and maybe an Am chord also. This tells us that the song is in the key of C, not so much because the song starts on C but because if the relationship of the chords, if the song started on the F chord it is still in the Key of C. another example would be a song with G C D and maybe Em, this song would be in the key of G because of the relationship of the chords.
Let’s use the C Major scale as it suits the purpose best.
C Major Scale: C D E F G A B C Scale Degrees: 1 2 3 4 5 6 7 8 Roman Numerals: I II III IV V VI VII VIII
All major chords are made up of the 1st, 3rd and 5th degrees of a scale. This would give us in the Key of C: C E G This is known as a tonictriad. The C is the root note of the scale and chord. It is also the tonic note. C E G is a C major tonic triad. ‘Tonic’ since it has the C as the first note and ‘triad’ since it has three (tri) notes in it.
When we talk of chords built on the different degrees of a scale we use Roman numerals.
The most important and best sounding chords that blend in with the C Major scale are chords that are built on the IV & V degrees of that scale. This gives us chords built on F(IV) and G(V).
A chord/tonic triad built on the IV i.e. F would be F A C (F A C being the 1st, 3rd and 5th degrees of an F scale) the chord/tonic triad would be an F Major chord.A chord/tonic triad built on the V i.e. G would be G B D (G B D being the 1st, 3rd and 5th degrees of the G scale) the chord/tonic triad would be a G Major chord.
The reason these two chords are so suited to C is that they both have notes in common with the C Major chord. F(IV) has the note C in common and G(V) has the G note in common. These common notes make the changes between the chords easy on the ear. Both the IV and V are major chords.
The next most likely chord to be used is one that is built on the VI degree i.e. A and the 1st 3rd 5th of the A scale is A C# E, well we shouldn’t have a C# in a in it as there is not a C# in the C scale. The C# must become a C and would then be A C E and this is an A minor chord. So in any key, a chord built on the VI of the scale needs to be a minor chord. This follows with all chords built on the notes of a scale, they must conform to the C scale.
All chords we wish to play in a key should be built on the notes of that scale. This applies to any key we play in i.e. in the key of A we use only the notes of the A scale. In the key of G only the notes of the G scale.
Below is a chart with the C major scale and the different chords that can be built on each degree of the scale, the note found in each chord, and the type of chord.
All other Keys have the same association of chords i.e. the II, III & VI should always be a minors. The VII will always be a minor b5.
It isn't essential that this method be used and many songs have been written using whatever chords the writer likes.
It does however give us the theory behind compatible chords that can be used as additional or passing chords within a song.
Chords built on the scale degrees
Scale Degree Tonic Triad Chord Type
C I C E G C (Major)
D II D F A Dm (Minor)
E III E G B Em (Minor)
F IV F A C F (Major)
G V G B D G (Major)
A VI A C E Am (Minor)
B VII B D F Bmb5 (Minor b5)
C VIII C E G C (Major)
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