Secondary Dominants

The strongest pull in music is the dominant chord leading home to the tonic. But composers found a trick: you can aim that same strong pull at a chord other than the tonic, making it feel, just for a moment, like the home chord. The chord that creates that pull is called a secondary dominant.

Think of it as borrowing the "leads home" feeling and pointing it somewhere new. For a moment, a chord in the middle of the key gets treated like a little temporary home, with its own dominant leaning into it. Then the music moves on. It adds a burst of extra pull and color.

Play a G chord, then a C chord, and feel the pull home. Now imagine aiming that same kind of pull at some other chord instead. That is what a secondary dominant does.

Secondary dominants take the central relationship of tonal harmony, the dominant resolving to its tonic, and apply it to chords other than the key's tonic. Any chord that can act as a momentary goal gets its own dominant a fifth above it, so the strong leading-tone pull that normally aims only at the tonic can now aim at a IV, a ii, a vi, or a V. This gives the leading-tone pull far more chords to aim at.

Tonicization is the concept underneath the label. For a moment, the ear treats the target chord as a local tonic, hearing the secondary dominant as its dominant, before the passage returns to the actual key. That momentary local home is the effect every secondary dominant produces.

The most common secondary dominant is called "five of five," written V/V. It is the dominant chord of the dominant, the chord that pulls strongly into the V chord. Because the V chord is so important, giving it its own dominant to lead into it is a favorite move.

In the key of C, the V chord is G. The dominant of G is D major, so D major becomes the five of five, pulling into G. Notice that D major needs an F-sharp, a note not in C major, and that sharpened note is exactly what creates the strong leading-tone pull into G.

Play a D major chord and let it resolve up to a G chord. That D pulling into G is the five of five.

The V/V earns its prominence because the chord it tonicizes, the dominant, is the key's most important chord after the tonic. Approaching the V through its own dominant intensifies the arrival on the dominant, which is why the progression V/V to V to I is a standard, strong way to reach a cadence. It drops a small dominant-tonic resolution right ahead of the cadential one.

Why the ear needs that sharpened note is the point. Scale degree four raised becomes the leading tone of the dominant, a half step below it, so it pulls up into the V the way the key's own leading tone pulls up into the tonic. Recognizing a raised fourth resolving upward is usually the fastest way to catch a V/V.

The five of five is just one example. You can build a secondary dominant for almost any chord in the key by the same recipe: find the chord you want to lead into, then build a major chord a fifth above it to act as its dominant. That new chord will usually need an accidental, a sharp or a natural sign that raises one of its notes.

So there is a whole family of these chords: the dominant of ii, the dominant of vi, the dominant of IV, and so on, each written with a slash, like V/ii or V/vi. Each one aims a strong pull at its particular target chord and then resolves into it.

Pick ii, iii, V, or vi in a major key, then build a major chord a fifth above it. That higher chord is the secondary dominant that leads into your chosen one.

A dominant must be a major triad with a leading tone into its own tonic, and that constraint is the whole recipe. To tonicize a chord you supply its leading tone, the note a half step below its root, which for most targets means adding a chromatic accidental. The same operation applies to every target, so the whole family shares one construction.

Not every chord can be tonicized. A secondary dominant needs a stable chord to resolve into, so the diminished triad on the seventh degree, unstable and lacking a perfect fifth, is not tonicized in the normal way. Only the major and minor triads of the key can stand in for the tonic, so only those take a secondary dominant.

There is a close cousin of the secondary dominant that does a similar job. Instead of borrowing a whole dominant chord to lead into a target, you can borrow just a leading-tone chord, the tense diminished chord that sits a half step below a note and pulls up into it. Aimed at a chord other than the tonic, it is called a secondary leading-tone chord.

It works much like a secondary dominant, creating a strong pull into a target chord, and it also uses a borrowed accidental. The two are relatives that both tonicize a chord; the leading-tone version simply uses a diminished chord to do it instead of a full dominant.

Think of a tense, spooky diminished chord sitting a half step under a target note, itching to resolve up into it. Aimed at a non-tonic chord, that is a secondary leading-tone chord.

Secondary leading-tone chords come from the same idea as secondary dominants: borrow the mechanism that resolves to a tonic and aim it at another chord. A dominant seventh and the leading-tone chord above it share the same tritone and the same resolving tendency, so a secondary leading-tone chord tonicizes its target much as a secondary dominant does, with the root left off. A vii-diminished of a target is that target's dominant seventh minus its root.

The fully diminished seventh form is the one composers reach for most. Its four notes are stacked in equal minor thirds, dividing the octave symmetrically, so a single shape can resolve into many different targets with only a respelling. Because any of its four notes can be spelled as the leading tone, that one shape can lead into four different roots, each a minor third apart.

Secondary dominants and their leading-tone cousins add spice and momentum to music without changing the key. They insert a moment of extra pull, a splash of a note from outside the key, that makes the arrival on the target chord feel stronger and more colorful. Then the music carries on in its original key as if nothing had happened.

The way to spot one is to look for a surprise accidental, a sharp, flat, or natural that does not belong to the key, that then leans by a half step into the chord right after it. Most of the time it is a raised note acting as a borrowed leading tone that pulls up into the next chord; the dominant seventh of IV is the exception, where its added lowered note slips down instead. Once you learn to see them, you find them everywhere.

Scan a piece for a stray sharp or natural that leans up by a half step into the root of the next chord. You have likely found a secondary dominant at work.

Dominant function can reach past the tonic without the passage ever leaving its key. By tonicizing internal chords, a secondary dominant adds chromatic color and forward drive, letting harmony visit a series of momentary local tonics while the underlying key holds firm. Secondary dominants are a common kind of chromatic chord in tonal music.

Distinguishing tonicization from modulation is the analytical judgment they demand. A secondary dominant tonicizes briefly, often for just a chord or two, before the home key returns. A modulation stays: it lands a cadence in a new key. The same chord can start either road, so you often cannot name it until the following bars resolve or fail to.