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A secondary dominant is a chord that functions as the dominant (or dominant seventh) of a diatonic chord other than the tonic. For example, V/V ('five of five') is the major triad or dominant seventh built on the second scale degree, resolving to V by treating V temporarily as a local tonic. Any diatonic triad except the diminished vii° can be preceded by its own dominant, creating chromaticism while remaining within the logic of tonal function. Secondary dominants are notated V/x or V7/x, where x is the chord being tonicized, and are the most common source of chromatic chords in common-practice music.
Learn V/V first, since it appears most frequently and is easiest to hear. Play V/V–V–I progressions in several keys to internalize the sound of the chromatic approach tone. Then learn V/ii, V/IV, V/vi by working through harmonizations that include them. Analyze pop and folk songs for secondary dominants — they appear more often than students expect.
You already know that V resolves to I because of the strong pull from the leading tone up to the tonic and from the seventh of V7 down to the third of I. Secondary dominants apply that same dominant-to-tonic logic to any diatonic chord, not just the tonic. The idea is simple: if V has special power to resolve to I, then the dominant of V has special power to resolve to V — and we can write that chord explicitly to intensify the approach to V.
The most common secondary dominant is V/V ("five of five"). In C major, V is G. The dominant of G major is D major (or D7). When you insert a D major chord before your G chord, you are briefly treating G as a temporary tonic. The F# in the D major chord — chromatic to C major — is the leading tone of G, and its upward pull to G is exactly what creates the intensified arrival. Play C–D7–G7–C at a piano and you will hear this immediately.
The notation V/x means "the dominant of chord x." Any diatonic triad except the diminished vii° can be tonicized this way: V/ii, V/iii, V/IV, V/vi, V/vii° would all be grammatical (though some are rarer than others). Each one introduces exactly one chromatic pitch — the raised leading tone of the target chord — and that chromatic tone resolves by half step when the target chord arrives. The resolution is what defines the function. A chromatic chord that doesn't resolve as expected is not functioning as a secondary dominant.
One subtlety worth internalizing: V/IV is the only secondary dominant that requires a *lowered* pitch (Bb in C major) rather than a raised one. This is because IV is a subdominant chord whose dominant seventh involves a flatted seventh scale degree. Students often expect secondary dominants to always raise a pitch, so V/IV surprises them. The logic is identical, but the direction of chromaticism reverses.
Secondary dominants are everywhere in tonal music — in hymns, folk songs, classical repertoire, and popular music alike. Once you learn to hear the characteristic chromatic approach tone and its resolution, you will start noticing secondary dominants in music you have known for years. They are the most common way composers add color and forward momentum to a diatonic harmonic framework without leaving the key.
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