Secondary Dominants (V/x)
A secondary dominant (also called an applied chord) is a dominant-function harmony that targets a chord other than the tonic. The notation V/x — read "five of x" — means the chord is built as though x were a temporary tonic: it uses the leading tone of x's key, which is almost always an accidental in the home key. The same principle extends to secondary leading-tone chords: viiº/x and viiº⁷/x work identically, borrowing the diminished-seventh framework from x's key.
Constructing a secondary dominant is straightforward: identify the dominant of the target chord, then raise its third by a half step (if it isn't already raised) to create a strong leading tone pulling toward x. For example, V/V in C major is a D-major triad or D–F♯–A–C dominant seventh — the F♯ acts as the leading tone to G (V). Because the accidental is not diatonic, secondary dominants signal an immediate and direct change in harmonic color, briefly placing the listener in x's orbit before returning to the home key.
Secondary dominants are one of the most common chromatic devices in tonal music. They intensify motion toward any major or minor chord that can plausibly act as a temporary tonic — V, IV, ii, vi, and iii are the most frequent targets. The effect is one of brief tonicization rather than full modulation: the home key remains audible, but the secondary dominant lends local weight and momentum to the chord it precedes.
