It is read “five of five”, the dominant of the dominant. In C major the dominant is G, so V/V is the chord a fifth above G: D major, or D7 with the C added.

Can a secondary dominant be inverted?

Yes, and its figures read as on any chord. V⁶₅/V puts F♯ in the bass of D7, V⁴₃/V puts A there and V⁴₂/V puts C. The chord still points at the same target; only the lowest note changes.

Is a secondary dominant a modulation?

Not usually. It touches another chord for a moment and the music returns to the original key at once. A modulation settles into the new key and stays there.

Secondary Dominants: V/V, V7/vi Explained

A secondary dominant is the dominant of a chord other than the tonic, used for a moment as if that chord were home. V/V in C major is D–F♯–A, the dominant of G, and it adds one accidental, F♯.

What is a secondary dominant?

Most chords in a key can be treated as a temporary home. The dominant of such a chord is the major chord a fifth above it, and played just before it, that chord pulls toward the target the way the ordinary V pulls toward I. That is a secondary dominant. It is written as a fraction: V/V is the dominant of V, V/vi the dominant of vi, and V7/IV the dominant seventh of IV.

I–V/V–V–I in C: D–F♯–A is the dominant of G, and its F♯ leads up to G when the chord resolves.

It resolves like any dominant: the root falls a fifth to the target, and the third of the chord, which is the target’s leading tone, rises a half step to the target’s root. A diminished triad built on that leading tone, such as vii°/V = F♯–A–C, leans toward the target in the same way. Like any chord, it can be inverted: V⁶₅/V is D7 with F♯ in the bass.

Which chords can have a secondary dominant?

Any major or minor triad of the key except the tonic can be a target: ii, iii, IV, V and vi in a major key; in a minor key the usual ones are III, iv, V and VI, the key’s own triads (V with the raised leading tone). Here are the five of C major, written as seventh chords, with the notes that are not in C major.

Target	Numeral	Chord in C	Outside C major
ii	V⁷/ii	A7	C♯
iii	V⁷/iii	B7	D♯, F♯
IV	V⁷/IV	C7	B♭
V	V⁷/V	D7	F♯
vi	V⁷/vi	E7	G♯

How do you spot one?

Look for an accidental the key signature does not give. In C major an F♯ points to V/V, a G♯ to V/vi, a C♯ to V/ii, a D♯ to V/iii and a B♭ to V/IV.
Check the kind of chord: a major triad or a dominant seventh sitting where the key would normally have a minor triad or no seventh.
See where it goes. A secondary dominant normally moves down a fifth to the chord it belongs to: A7 to Dm, B7 to Em, D7 to G, E7 to Am.

Secondary dominants