Secondary dominants

Every major and minor key has one dominant: the chord on the fifth degree, which pulls back to the tonic. A secondary dominant applies that same pull to a different target. Instead of only V wanting to reach I, you build a dominant aimed at ii, or iii, or vi, and that chord becomes a temporary destination.

The notation

Secondary dominants are written with a slash and read as "of":

• V/V — "five of five", the dominant of the dominant
• V7/vi — "five seven of six", the dominant seventh aimed at vi

The chord on the left is the function. The chord on the right is the target.

How to build one

Take the target chord. Treat its root as a temporary tonic. Build a dominant seventh a fifth above it.

In C major, to build V/V:

1. The target is V, which is G.
2. A fifth above G is D.
3. The dominant seventh on D is D7 — D F♯ A C.

1. CITonic
   →
2. D7VDominant
   →
3. GVDominant
   →
4. CITonic

The same recipe gives the rest:

Why it pulls

The pull comes from a note that does not belong to the key.

D7 contains F♯. C major has no F♯. That F♯ is the leading tone of G — a semitone below it — and a leading tone is the strongest directional signal in tonal harmony. For as long as F♯ is sounding, the ear is being told that G is home.

This is why a secondary dominant sounds like a brightening or a sharpening rather than a mistake. The foreign note is not random; it is precisely the note that would exist if the target chord were the tonic.

What it is not

A secondary dominant is not a key change. The visit is momentary — usually one chord, resolving immediately — and the original key resumes without ever having been abandoned. Theorists call that brief visit tonicization: the target is treated as a tonic for a moment.

Modulation is the larger move, where the new key is established well enough that it becomes the reference point. The line between them is a matter of degree and context rather than a rule, and analysts disagree about individual passages. If a cadence confirms the new key and the music stays there, it is a modulation; if one chord glances sideways and comes straight back, it is a tonicization.

Where you will hear them

V/V into a half cadence is one of the most common gestures in classical music: the phrase leans on D7, arrives on G, and stops there, open. In popular music V/vi and V/ii turn up constantly — the E7 in a C major song that makes the arrival on Am feel earned rather than merely next.

Reading them in a progression

When you meet an unexpected major or dominant seventh chord in an otherwise diatonic progression, check whether the chord a fourth above it appears next. If it does, you are almost certainly looking at a secondary dominant, and the foreign note will be the leading tone of that next chord.

If it does not resolve where it points, the label is doing less work, and it may be better described as a borrowed or chromatic chord instead.