Tonicization is the process of momentarily emphasizing a non-tonic chord by using chords borrowed from the key in which that chord is tonic. Unlike modulation, there is no cadence in a new key, only a short progression of chords borrowed from another key.

The chord that is tonicized is typically a chord that belongs to the present key. The chords that emphasize it are usually the chords borrowed from another key. And these chords are usually chromatic alterations of chords native to the present key.

The most straightforward example is when a subdominant chord is chromatically altered by changing fa to fi, and then progresses, like usual, to the dominant chord. This alteration of fa to fi turns a regular subdominant chord into a chord that has a dominant function in the "key of the dominant."

For example, take the chord progression F–G–C in C major. We would label this progression as IV–V–I with Roman numerals. If we change fa (F) to fi (F#) in the F chord, we get F#dim–G–C. In the key of C, we might analyze this progression as #ivº V I, noting the change in root and quality. However, we can also note that F#-dim is native to G major; it is a dominant-functioning chord (viiº) in the key of G—the key in which the following chord is tonic. In other words, we are borrowing the dominant chord from the key of G and applying it to the G-major triad. Thus, we can re-interpret the F#dim as an applied dominant of the G chord, which we label viiº/V—read "seven of five." Thus this progression is labeled viiº/V V I.

Note, however, that though we "borrowed" or "applied" a dominant chord from G major, that chord is acting in the context of C major. Its scale degrees are fi, la, and do: an alteration of scale degrees that clearly express subdominant function. Thus, in C major, the progression F#dim–G–C still expresses the functional progression S–D–T. (In functional bass—see below—this progression would be analyzed as [S+4] D5 T1.)

Exercise: change one note in the following excerpt to turn the subdominant chord into an applied chord. What Roman numeral and functional bass would you give that chord?

Such borrowing of chords can happen for any major or minor triad in the home key. Any diatonic triad can take an applied chord—a chromatically altered chord that also functions as a dominant chord in which the following chord is tonic. Thus, the example progression of F#dim–G can occur in any key to which the G-major triad belongs: G major, C major, D major, B minor, A minor, or E minor.

Two things will always be true of the applied chord:

• The chromatically altered chord will function as a dominant chord in the key of the chord that follows it (V or VII).
• The chromatically altered chord will be an alteration of the function that logically precedes the function of the chord that follows it.

On the latter point, if the tonicized chord has tonic function in the current key (such as mi–sol–ti or la–do–mi), the applied chord will be an altered dominant of the current key. If the tonicized chord has dominant function in the current key, the applied chord will be an altered subdominant of the current key. If the tonicized chord has subdominant function in the current key, the applied chord will be an altered tonic of the current key.

applied chord		tonicized chord
altered T	→	S
altered S	→	D
altered D	→	T

Analytical notation

Applied dominant chords will always make use of slash notation. Rather than #IV, #V, etc., in the Roman numerals, convention is to express its identity in the key that it is borrowed from. If in the key of E-flat, an F-dominant-seventh chord is used to tonicize a B-flat triad, we label the F7 chord as V⁷/V, rather than II⁷. This goes on the same line as the Roman numerals for diatonic chords.

Underneath the slash notation, we label its harmonic function (T, S, or D). Here we show the way the chord is functioning in the home key, not the key from which it is borrowed. So while F7 may be V/V in E-flat, it is a chromatically altered S, and so belongs in the S harmonic zone.

This distinction is important. Applied chords are employed within the context of a tonal phrase, and without a cadence in a new key (a modulation), these borrowed chords still play a role within the original key. So while we reflect their borrowedness via slash notation in the Roman numerals, it is also important to reflect their role in the broader harmonic context of the phrase by means of non-modulating functional labels.

Following is a short keyboard-style passage that includes two different applied chords, notated with Roman numerals.

Notes on functional-bass notation

Like chromatically altered subdominant chords, every applied chord will have two elements to its functional bass symbol. First, on the normal line of functional bass analysis will be a symbol showing its function in the current key and the scale degree of the bass note (with "+" or "–" for altered bass pitches), surrounded by square brackets to signify that the chord is chromatically altered. (The square brackets are necessary no matter in which voice the chromatic alteration occurs.) Second, below the normal line of functional bass analysis will be a symbol denoting the key from which it is borrowed and the functional bass symbol the chord would have in that key.

In our above example of F#dim–G–C in the key of C, the regular functional bass line would read [S+4] D5 T1, and below the [S+4] would be the symbol D7/V. The latter symbol uses a slash to denote "in the key of" and a Roman numeral to denote the tonic of that key relative to the current key. We will use Roman numerals similarly when studying modulation to denote tonics of key areas to which the music modulates. Roman numerals, however, are never used to denote chordal roots in the context of a functional bass analysis.

Here is the same example above, but with the top layer of functional bass analysis replacing the Roman numerals. (Trinket only supports one layer of lyrics/harmonic analysis.) What chords would require a second layer of functional bass analysis? What would those symbols be?

(You can find the interpreted functional bass version here.)

Scale degrees in applied chords

Just as the various dominant functioning chords in a key will contain some combination of the dominant-functioning scale degrees—sol, ti/te, re, fa, and/or la/le (usually le)—each category of applied dominant chords will have their own set of usual scale degrees. They are as follows.

D/II — di, mi, sol, la, and te.
D/lowered-III (minor) — re, fa, le, and te.
D/III (major) — ri, fi, la, ti, and do.
D/IV — mi, sol, te, do, and ra.
D/V — fi, la, do, re, and me.
D/lowered-VI (minor) — sol, te, ra, and me.
D/VI (major) — si, ti, re, mi, and fa.
D/lowered-VII (minor) — la, do, me, and fa.

Functional dissonances in applied chords

Since tonicization temporarily borrows a chord from another key, and since that chord is made up of scale degress from that other key, an applied chord involves borrowing scale-degree tendency from that other key. In other words, in an applied chord, judge functional consonances (triggers and associates) and dissonances, as well as tendency tones like ti, relative to the key borrowed from, not the home key.

As a shortcut, that usually means the seventh of a seventh chord or the fifth of a diminished chord are functional dissonances. (In a half- or fully-diminished-seventh chord, both the fifth and seventh tend to be functional dissonances.) Also as a shortcut, the chromatically raised tone tends to be the leading-tone of the tonicized key.

In functional bass, use the functional label before the slash and the Roman numeral after the slash to determine functional dissonances more reliably. Consider a D5/V (altered S2) chord in G major. In the home key, the chord contains re (A), fi (C-sharp), la (E), and do (G). But those pitches are sol, ti, re, and fa in the key of V (D major). Thus, the C-sharp should be treated like a leading-tone, and the G (fa in D) like a functional dissonance of dominant function.

Exercise: In the examples above, find the functional dissonance in each applied chord. What pitch is it? How does it progress into the next chord?

Exercise: on the bullet list above, determine which solfège syllable(s) are the functional dissonance for each applied chord.