SECONDARY CHORDS

Before explaining the secondary chords, we shall first explore the closely related keys.  

Closely related keys

Two keys are closely related if their key signatures differ in one accidental. Such two keys are adjacent in the circle of fifths (see chapter Circle of fifths).

For instance, the C major key (its key signature has no accidentals) has the following closely related keys:  

• the A minor key (its key signature also has no accidentals) which is the relative minor of the C major key

• the F major key (its key signature has one flat)

• the D minor key (its key signature also has one flat)

• the G major key (its key signature has one sharp)

• the E minor key (its key signature also has one sharp)

If we take the diatonic triad chord on the first scale degree of any of these keys, we get a diatonic triad chord of the C major key. Thus, each current key’s closely related key has the diatonic triad chord on the first scale degree that is equal to one of the diatonic triad chords of the current key. The following two figures illustrate this for the C major key and the A minor key but the same holds for any major key and any minor key.

The next figure shows the diatonic triad chords in the C major key and marks those which are equal to the diatonic triad chords on the first scale degree of the closely related keys. Thus, d:i is the same chord as C:ii, e:i is the same chord as C:iii, F:I is the same chord as C:IV etc. Please note that while these chords are the same, the harmonies are different. For instance, C:ii and d:i make the same Dm chord but are considered different harmonies. 

Figure: The diatonic triad chords on the first scale degree of the closely related keys of the C major key

The next figure shows the diatonic triad chords in the A minor key and marks those which are equal to the diatonic triad chords on the first scale degree of the closely related keys.

Figure: The diatonic triad chords on the first scale degree of the closely related keys of the A minor key

Secondary chords and closely related keys

A secondary chord is a chord on the scale degree of a current key’s closely related key. Most often, it is a diatonic chord in the closely related key, but sometimes it can be an altered chord. A secondary chord is usually an altered chord in the current key (that is the reason why we are talking about secondary chords in the chapters related to altered chords), but it does not have to be. We shall see an example of a secondary chord which is a diatonic chord in the current key, in chapter  Bach’s Prelude No. 8 in Eb minor from Well-Tempered Clavier.

Annotating secondary chords

The following two figures show an example of a secondary chord and the method of annotating it. In this example, the current key is the C major key while the D dominant seventh chord appears. In the current key, this chord is an altered seventh chord on the second scale degree. The diatonic seventh chord on the same scale degree would be the minor seventh chord. However, the same chord is also the diatonic seventh chord on the fifth scale degree of the G major key. This is shown in the next figure.

Figure: An example of a secondary chord (not yet properly annotated) 

Since the G major key is closely related to the current key (the C major key), we say that the D dominant seventh chord is a secondary chord in the current key and can be annotated as V7/V, where V7 represents the dominant seventh chord on the fifth scale degree and V represents the current key’s closely related key on the fifth scale degree (that is, the G major key because its tonic G is the fifth scale degree of the current key). We read the slash symbol as “of”. That is, we read V7/V as “the dominant seventh chord on the fifth scale degree of closely related key on the fifth scale degree” or simply “five-seven of five”. This is shown in the next figure.

Figure:  An annotated secondary chord

Secondary chords usage

Composers often use secondary chords to create more expressive music. Two or more secondary chords from the same closely related key appear and form a chord progression in the closely related key. This is only a temporary change of the key and we do not annotate temporary changes of the key. As an example, we can examine the ending of Chopin’s Prelude No. 7 in A major.

Figure: The secondary chord in Chopin's Prelude No. 7 in A major

Here, we have the chords F#7 and Bm, which form the chord progression V7 - i in the B minor key. The B minor key is the closely related key of the A major key on the second scale degree. Thus, we can identify these two chords as secondary chords and annotate them as A: V7/ii - i/ii instead of b: V7 – i. Since i/ii is the same as ii (and thus, it is always annotated as ii) the end result is: A: V7/ii - ii. 

Commonly used secondary chords

The commonly used secondary chords are: secondary dominant chords, secondary leading-tone chords and secondary supertonic chords. The secondary dominant chords appear most often, out of those three. 

We shall explain each of them. Also, we shall give a few typical chord progressions in which they appear. The chord progressions will be given in the closely related key. Chord progressions that occur often will be examined in detail in chapter Typical chord progressions. Here, some of them are listed only to support easier noticing of secondary chords. 

Secondary dominant chords 

A secondary dominant chord is a chord on the dominant (fifth scale degree) of the current key’s closely related key. If the closely related key is a minor key, its seventh scale degree is usually raised. Thus, in both the major and minor closely related key, the secondary dominant chord is usually either the major triad chord or the dominant seventh chord. The secondary dominant chord usually appears in the following chord progressions (in the closely related key): V - I or V7 – I. We have already examined an example of the secondary dominant chord in Chopin’s Prelude No. 7 in A major.

Secondary leading-tone chords

A secondary leading-tone chord is a chord on the leading-tone (seventh scale degree) of the current key’s closely related key. If the closely related key is a minor key, its seventh scale degree is raised (to get the leading-tone instead of the subtonic). Thus, the secondary leading-tone chord usually appears in the following chord progressions (in the closely related key): viio – I or viiØ7 – I when the closely related key is a major key, or viio – i or viio7 – i when the closely related key is a minor one.

Secondary supertonic chords

A secondary supertonic chord is a music chord on the supertonic (second scale degree) of the current key’s closely related key. The secondary supertonic chord often appears in the following chord progressions (in the closely related key): ii - V – I or ii – V7 – I when the closely related key is a major one, or iio – V – i or iio – V7 – i when the closely related key is a minor one.

Isolated secondary chord

Sometimes, we may come across a secondary chord which is isolated. That is, it is not a part of any chord progression in the closely related key. If possible, it is best to not annotate such a chord as a secondary chord. The reason is because annotating it as a secondary chord does not explain harmony any better.

Bach’s Prelude No. 8 in Eb minor from Well-Tempered Clavier, book I

As an example of a secondary chords usage, we can examine the beginning of Bach’s Prelude No. 8 in Eb minor from Well-Tempered Clavier, book I. Here, we have a few secondary dominant chords.

The first case is the Gb major triad chord in the first inversion that appears in measure 5. Note that this chord can be represented in the Eb minor key in the following two equivalent ways: as III6 or as V6/VI (which is V of the Cb major key, where the Cb major key has seven flats and is enharmonically equivalent to the B major key, as it was shown in chapter Circle of fifths). However, since after the seventh chord variant of the same chord, the next chord is the Cb major triad chord, which is VI in the Eb minor key, it is more instructive to show the Gb major triad chord as V6/VI. This way, we are showing that we have the chord progression from V to I in the Cb major. 

The second case is the Eb major triad chord in the first inversion that appears in measure 7. Note that this chord can be represented in the Eb minor key in the following two equivalent ways: as I6 borrowed from the parallel major or as V6/iv (where the seventh scale degree of the Ab minor key is raised). Again, it is more instructive to show this chord as V6/iv. This way, we are showing that we have the chord progression from V to i in the Ab minor key. Also, we are showing that we have a sequence of two similar chord progressions, one after the other: from V to I in the Cb major key and from V to I in the Ab minor key. When we talk here about the Cb major key and the Ab minor key, we consider them as temporary changes of the key and we do not annotate temporary changes of the key. Thus, only Eb minor key is annotated here. This will be further explained in chapters Tonicizations and Modulation compared to tonicization.  

Figure: Bach's Prelude No. 8 in Eb minor from Well-Tempered Clavier, book I