Dominant of something else
In a major key, the normal dominant is V and resolves to I. A secondary dominant temporarily treats another diatonic chord as if it were a tonic, then places that chord’s own V or V7 immediately before it.
V/V → V → I
D7 contains F♯, which is outside C major. That F♯ acts as a temporary leading tone to G, making the arrival on the real dominant sound stronger.
How to find a secondary dominant
- Choose the diatonic chord you want to target.
- Treat its root as a temporary tonic.
- Find the major chord or dominant seventh a perfect fifth above that root.
- Resolve it to the target chord.
V/vi → vi
E minor belongs naturally to C major, but E7 contains G♯. The raised note pulls upward to A and makes Am sound temporarily central.
Reading the slash notation
Read V/ii as “five of two,” not as a slash chord. The numeral after the slash is the target. In C major, ii is D minor; the dominant of D is A or A7, so A7 is V/ii.
Common secondary dominants in C major
A7 can lead to Dm (V/ii), B7 to Em (V/iii), C7 to F (V/IV), D7 to G (V/V) and E7 to Am (V/vi). The diminished vii° chord is rarely tonicised because it is not a stable major or minor target.
Resolution can be delayed
The expected resolution is the clearest way to learn the sound, but composers sometimes interrupt or extend it. First learn to hear the raised leading tone and intended target; then experiment with deceptive motion.
Secondary dominant or key change?
A single tonicisation is brief: the original key still feels like home. A modulation establishes the new key more strongly through duration, cadences or repeated functional motion. Context, not the chromatic chord alone, tells you which one occurred.