Understanding Chords in Relation to Key Signatures: A Comprehensive Guide

Understanding chords and key signatures is critical for grasping music theory. This article delves into their relationship, explaining how chords are built from scales and how they relate to key signatures. We will explore diatonic and non-diatonic chords and the practical implications of modulation in music.

Key Signatures

A key signature indicates the set of sharps or flats used consistently throughout a piece of music. It defines the tonality of a piece, indicating whether it is in a major or minor key. Each key signature corresponds to a specific scale:

C Major

No sharps or flats

G Major

1 sharp: F

D Major

2 sharps: F, C

A Minor

No sharps or flats (relative minor of C Major)

E Minor

1 sharp: F

Chords and Their Relation to Key Signatures

Chords are built from the notes of a scale. In a given key, certain chords are considered diatonic, meaning they are formed using only the notes from the scale associated with that key. For example:

In C Major:

I: C - E - G ii: D - F - A iii: E - G - B IV: F - A - C V: G - B - D vi: A - C - E vii°: B - D - F

Do Chords Change?

While the diatonic chords are consistent within a key signature, composers often use non-diatonic chords or borrowed chords from other keys to create tension, add color, or create interest. For example, the D7 chord (D - F - A - C) can be used in C Major even though F is not in the C Major scale. This chord introduces a dominant function that resolves to G Major (the V chord in C Major).

Harmony and Modulation

The key signature is the specification of sharps and flats needed to play in a key. However, the key itself has strong affinities with particular chords, which are the same even in non-traditional keys.

Major Keys:

Principal chords: I, IV, and V (Tonic, Subdominant, Dominant) II and vi (Super tonic and Submediant) ii, iii, and vii° (Diatonic minor chords)

Minor Keys:

In the natural minor, the chords are identical to their relative major. However, in the harmonic and melodic minors, additional chords can be used, giving more flexibility.

In the harmonic minor, the minor V is replaced with a major V, and augmented chords can be used as well.

In the melodic minor, the chords are the same as the relative major descending, but with a major IV and V. This allows for many more options for chords.

Modulation:

In classical music and jazz, modulating away from the home key is common. For example, modulating to the dominant (V) is so common that it’s often done without leaving the tonal framework. Similarly, modulating to the relative minor (iii) allows for the introduction of a major III.
Other obvious modulations include going down the circle of fifths to the key based on the IV, making the vii° a VII major. Exploiting the diminished and augmented chords can lead to virtually any key.

Conclusion

Chords fit into key signatures based on the notes available in the scale. Diatonic chords are those that naturally occur within the key, while non-diatonic chords can be used to add variety and tension. Understanding these concepts helps you see how harmony works in music and how composers manipulate chords to create different effects.