Secondary Dominants and How Tonicization Works
A secondary dominant makes any chord a temporary tonic. How to spell one, where it turns up, and what cancels the effect.
In C major, a D major triad contains an F sharp. The key signature does not supply it and the chord is not an error. For as long as it sounds, the music treats G as a temporary tonic, and the D major triad is the dominant of that temporary tonic. Analysis writes it V/V and reads it “five of five”. The device is common enough that a page of Bach or Mozart will usually contain one.
Tonicization, not modulation
A secondary dominant is the audible half of tonicization: a chord other than the tonic is raised, for a moment, to the status of a local tonic. The raising is not a decision taken by the analyst. It is what the ear does when a chord arrives by its own dominant, and the signal is the leading tone. In C major the leading tone is B. When another note starts behaving like a leading tone to a note that is not C, a tonicization is under way.
Two qualifications keep the term honest. First, a tonicization operates at the level of a chord or a short progression, not a key. Nothing has been confirmed by cadence, and the phrase keeps its original direction once the chromatic note resolves. Second, the chord being tonicized is normally diatonic in the home key. V/V, V/ii, V/vi and V/iii in major, V/iv and V/III in minor, are the working set. A chromatic chord can be tonicized, but the labelling quickly becomes unreadable.
The difference between a tonicization and a modulation is a difference in what happens afterwards. A tonicization resolves and is forgotten. A modulation is confirmed by a cadence in the new key, and the new key persists.
How to spell one
The procedure is mechanical, and it is worth doing on paper before it becomes automatic.
- Name the chord you intend to tonicize. Take ii in C major, which is D minor.
- Treat the root of that chord as a temporary tonic: D.
- Build the dominant of D, a major triad rooted a fifth above: A, with the notes A, C sharp, E.
- Spell the chord with the leading tone of D, whether or not the home key contains that note.
The chromatic note is the entire point. In a major key the note that creates each secondary dominant is predictable, because it is the leading tone of the target raised out of the home scale.
| Figure | Chord in C major | Notes | Resolves to | Chromatic note |
|---|---|---|---|---|
| V/V | D major | D–F sharp–A | V (G–B–D) | F sharp, raised 4 |
| V7/V | D dominant seventh | D–F sharp–A–C | V | F sharp |
| V/ii | A major | A–C sharp–E | ii (D–F–A) | C sharp, raised 1 |
| V/vi | E major | E–G sharp–B | vi (A–C–E) | G sharp, raised 5 |
| V/iii | B major | B–D sharp–F sharp | iii (E–G–B) | D sharp, raised 2 |
| V/IV | C major | C–E–G | IV (F–A–C) | none |
The last row is the interesting one. V/IV is spelled note for note like the tonic. A C major triad can only be heard as the dominant of F if the music then reaches F, and by that point most analysts simply write I and let the progression carry the meaning. The label is a statement about resolution, so it is earned by the resolution and not by the spelling.
In a minor key the arithmetic shifts. In C minor, the leading tone is already raised by convention, so the chromatic note comes from elsewhere: raising the third degree, E flat to E natural, produces the C major triad that acts as V of F minor, and raising the seventh degree, B flat to B natural, produces the G major triad that acts as V of C minor itself.
Where they cluster
Three situations produce secondary dominants more often than the rest.
Before a dominant. A phrase that would simply move IV–V can move IV–V/V–V instead, which makes the dominant feel earned rather than stated. This is the most common use in classical repertoire, and it very often precedes a half cadence, since V/V points at V and V points at nothing.
In sequences. Chained secondary dominants, each resolving down a fifth to the next, produce the circle-of-fifths motion that sits behind a great deal of tonal music. In C major, A7–D7–G7–C is V/ii, V7/V, V7 and I, and every chord is heard as the dominant of the one after it.
In the approach to a chromatic chord. A borrowed chord arriving from a secondary dominant sounds purposeful rather than abrupt. The same machinery also supplies the dominant for chords that the key signature cannot support, which is why V/iv turns up in minor-key passages with an F minor chord in them.
The chain is also where the notation strains. When each chord in a sequence is the dominant of the next, labelling every step V of something describes the mechanics correctly and says nothing about the passage’s key, which has not changed at all.
What cancels a tonicization
The chromatic note resolves, the raised degree returns to its diatonic form, and the phrase continues. That return is the cancellation. If the chromatic note resolves upward by semitone into the target chord and the target chord then behaves as a tonic for a full phrase, with its own dominant and its own cadence, the process has become a modulation instead, and the label should change accordingly.
The distinction matters for analysis because the two devices look identical at the moment of arrival. A V/V and the dominant of a new key are the same chord. What separates them is the cadence that does or does not follow.
Part-writing cautions
The chromatic note behaves as a leading tone and resolves up by semitone: the F sharp of V/V moves to G. It should not be doubled, and it should not leap away from its resolution. When the secondary dominant carries a seventh, as in V7/V, that seventh resolves down by step, so the C of an open D7 moves to the B of G.
Spell by function rather than by convenience. V/ii in C major is written with C sharp because the note is the leading tone to D. Written as D flat it would be describing a descent, and the resolution would no longer make sense.
A secondary dominant is a chord like any other, and the voice-leading rules that govern V govern V/x as well. Because the chords are usually in root position with the bass moving by fifth, parallel octaves between the bass and a doubled root are a standing temptation, and the resolution into the target chord is exactly where they tend to appear.
Dominant function does not require a root. In place of V/ii, the diminished seventh on the raised fourth degree, C sharp, E, G, B flat in C major, supplies the same pull towards D minor without a root position dominant. These substituted chords are common in minor keys and in sequences, where an even chord-to-chord motion is wanted.
Hearing the function
The raised note is the announcement. In a C major context, an F sharp means G is coming, a C sharp means D is coming, a G sharp means A is coming, and the ear registers this faster than the labels can be written down. Play I–V/V–V–I in C major and stop. Then play I–V/V–vi. The second version builds the same dominant and does not deliver the tonic it promised, and the surprise is audible as a harmonic event rather than as an analysis. That comparison, run in several keys, does more for recognition than any amount of labelling.
The ear training studio on this site includes a dominant-function drill for exactly that comparison.
