Now that we’ve learned about harmonic function and melodic embellishments, the last strategy that composers use to add interest and variety to a piece of music is tonicization or modulation, which are both processes of moving to different keys in order to create harmonic interest or change the mood of a piece. 

Tonicization is a really brief modulation of keys, where you just borrow a few notes from another key. The key that you are borrowing notes from is usually called the secondary key, and the tonic of the secondary key is called the temporary tonic. 

For example, in a piece of music in the key of C major, the tonicization of the key of G major would involve the use of chords and progressions that are characteristic of G major, such as the use of the G major chord, or the progression I-V-vi-IV, before resolving back to the C major chord.

Closely Related Keys 

Usually, the secondary key is a closely related key to the primary key, which is the key that the piece is written in. Closely related keys are musical keys that are closely related harmonically, meaning that they share many of the same pitches and chord progressions. These keys are often used in music to create smooth transitions between different sections of a piece.

There are several pairs of closely related keys:

1. Relative keys: Relative keys are keys that share the same key signature, meaning they have the same pitches, but have a different tonic, or root pitch. For example, the key of C major and the key of A minor are relative keys.
2. Dominant and subdominant keys: The dominant and subdominant keys are keys that are a perfect fifth apart. For example, the key of G is the dominant key of C, and the key of F is the subdominant key of C.
3. Tonic and dominant keys: The tonic and dominant keys are keys that are a perfect fourth apart. For example, the key of F is the dominant key of Bb, and the key of Bb is the tonic key of F.

Here's another way to think about closely related keys. First, a key that is next to another on the circle of fifths can show us whether a key is closely related or not. There are 5 closely related keys for each key signature. Look down at the key of A Major. Let's find all 5!

Image via Neel Modi 

We can see that its relative minor is f# minor. A key can be both a relative key AND closely related. What are the major keys on either side of A major? One is D Major. The other is E major. What are the relative minor keys of the major keys which just found? The relative minor of D Major is b minor. What is the relative minor of E Major? C# minor.

Note that a closely related key signature is only one accidental away from the original key. In the example we used before, A Major has three sharps, D Major has two, and E Major has four. Take the relative minor keys of the adjacent keys, and we add up the relative keys again to a total of 5.

Identifying Tonicizations in Music 

The best way to identify tonicizations is to look for accidentals from closely related keys in a passage. This is especially true if you notice an accidental (that is not the leading tone in minor) resolving upward by step. This is called a leading tone relationship, and it is likely that this note is resolving to a temporary tonic note. 

Now that you know the tonic of the secondary key, you can figure out which chord led into the temporary tonic. Most likely, this chord will have a dominant function, so it will either be a V chord or a vii chord in the secondary key. 

Let’s look at an example: 

Image via https://www.expandingthemusictheorycanon.com/extended-tonicization/ 

This is an excerpt from “Partant pour la Syrie” by Beauharnais. It was written in the late 18th century, meaning that it follows 18th-century voice leading rules. 

We are in the key of D Major. Notice that in Bar 3, there are lots of G#s popping up. This is a clue that we might be dealing with a tonicization. What is the secondary key? When leading from Bar 3 to Bar 4, we see the G# leading to an A, which gives us a hint that we might be an A Major. This makes sense, because A Major is a closely related key to D Major – it is only one step away on the Circle of Fifths. 

Next, we want to know what the chord preceding the A Major chord is. We have a G#, B, and D in the voices, and this chord is surrounded by some Es, so we might have a V7 chord in A Major. This also makes sense, since V7 chords carry a dominant function. 

When performing a contextual analysis of tonicizations, we won’t write this E7 chord as a II7 chord, because the chord doesn’t really function as a II7 chord within the piece. Instead, we will denote it as a V65/V chord. 

Where do we get these Roman numerals from? First, notice that the temporary tonic, A, is the fifth scale degree of our primary key, D Major. This is where we get the V. It is a V instead of a v because the secondary key is Major – not minor. The first Roman numeral comes from the fact that the chord built on G# is a V 6/5 chord in A Major. 

When talking about these chords, we read the notation V65/V as “V 6/5 of V,” meaning that we have a first inversion V7 chord in the temporary key of the dominant scale degree in the primary key. 

Secondary Dominants 

The reason why we usually have the chord leading into the temporary tonic as a dominant chord is that we want listeners to hear the temporary tonic as a tonic. Since the leading tone in dominant chords have a strong tendency to resolve to the tonic, the leading tone in these “temporary” dominant chords have a strong tendency to resolve to the temporary tonic. 

These temporary dominant chords have a special name: secondary dominants. 

The most common secondary dominant is the V/V, which is the dominant of the dominant chord. For example, in the key of C major, the dominant chord is G major, and the V/V is D major. By using the D major chord, the music temporarily tonicizes the key of G major before resolving back to C major.

Secondary dominants can also tonicize chords other than the dominant. For example, in the key of C major, the V/ii tonicization, which uses the A Major chord followed by the D minor chord, temporarily tonicizes the D minor chord. We can also have the secondary dominant of the subdominant, which would be a V/IV chord, and it would resolve to the IV chord. 

Notice that in C Major, the V/IV chord resolving to the IV chord won’t have extra accidentals, so it would be hard to tell if this is a I-IV progression or a V/IV-IV progression. Usually, though, it will be apparent from context if there is a temporary tonicization. This might include adding accidentals, specifically a Bb in C Major, in non-chord tones or surrounding chords, or it might involve using the subdominant tone more frequently surrounding the chord progression. 

Secondary dominants are usually used around cadences. Secondary dominants can also be extended to create longer chains of tonicization, known as "cadential extension." This can involve the use of multiple secondary dominants in succession, creating a sense of tension and release as the music temporarily tonicizes multiple keys before resolving back to the main key.

Cadential extension usually involves either adding suffixes or prefixes to the cadence, although developing the dominant and tonic sections of a cadence is also possible. Suffixes are harmonic extensions of the cadence after resolving to the dominant section, and usually involve tonicizing chords other than the dominant in order to prolong the section after a phrase. For example, you can have a V-I cadence, followed by a V/ii-ii cadence, and so on. 

These suffixes can either eventually resolve back to the home key, as in an ascending fourths harmonic sequence, or they can signal modulation to a new key. Note that it is always the case that these tonicizations will involve chromaticizations in order to ensure that the secondary dominant is a Major triad or Major-minor seventh chord. 

Prefixes, on the other hand, also have the same properties has suffixes in terms of the chromaticizations and the ascending fourths sequence, but these prefixes end with a cadence in the home key, since they are appearing before the final cadence. For example, a ii-V/IV-IV-V-I cadential extension is common. Notice that if we wrote this is a ii-I-IV-V-I cadence, we obfuscate the fact that the I chord is acting as a secondary dominant for the IV chord, and it looks strange that a ii is voice leading to a I chord. This is why contextual analysis is important.