A secondary dominant is a dominant-seventh chord applied to a chord other than the tonic. V7/V (five-of-five) is the most common. Learning to hear these borrowed dominants is essential to understanding chromatic harmony.
THE CONCEPT OF SECONDARY DOMINANTS
In diatonic harmony, the only dominant chord is V (and V7). But composers from the Baroque period onward discovered that any chord in the key can be briefly tonicized — treated as if it were a temporary tonic — by preceding it with its own dominant seventh chord. This is a secondary dominant: a dominant seventh chord temporarily applied to a chord other than the home tonic.
The notation is V7/x, read as "five-seven of x." V7/V means the dominant seventh chord of the dominant: in C major, the dominant is G, and the dominant of G is D, so V7/V = D-F#-A-C (D7). V7/IV = C-E-G-Bb (C7, the dominant seventh of F). V7/ii = A-C#-E-G (A7, the dominant seventh of Dm). Each of these introduces a note foreign to the original key (F# in V7/V, Bb in V7/IV, C# in V7/ii), creating a momentary chromatic inflection that adds colour and direction.
HEARING SECONDARY DOMINANTS
By ear, secondary dominants have a characteristic sound: a dominant seventh chord arriving "early," before the expected moment of cadence. The most recognizable is V7/V (the double dominant): when you hear a major chord with a minor seventh that resolves to the dominant chord (which then resolves to the tonic), you are hearing a secondary dominant.
The characteristic feature of any secondary dominant is a raised note — a chromaticism that is a half step below the chord it precedes. In C major, V7/V (D7) contains F#, which is a half step below G (the target dominant). This leading tone quality is exactly what makes secondary dominants so powerful: they apply the same tension-resolution mechanism of the primary dominant to any chord in the key.
DISTINGUISHING SECONDARY DOMINANTS FROM MODULATION
The crucial distinction in ear training: a secondary dominant momentarily tonicizes a chord without leaving the key; a modulation establishes a new key centre that persists for multiple phrases. By ear, the distinction is duration and subsequent context. If the "new tonic" lasts only one or two beats and the original tonic reasserts itself quickly, you have heard a secondary dominant. If the new tonic lasts through multiple phrases, with cadences confirming it, you have modulated.
In practice, there is a gray area — experienced musicians sometimes disagree about whether a passage is a brief tonicization or a genuine modulation. The pedagogical guideline is: tonicization if the original key returns within the same phrase or sentence; modulation if the new key needs its own cadences (sometimes a new key signature) to establish itself convincingly.
COMMON SECONDARY DOMINANT PATTERNS
The most common in tonal music: V7/V → V → I (the double dominant approach to a perfect authentic cadence — extremely common in Classical music); V7/IV → IV (moving from tonic area to subdominant with extra momentum — common in hymns and blues); V7/ii → ii (approaching the supertonic chord with a secondary dominant — creates smooth chromatic motion in the bass).
In jazz, secondary dominants are used constantly, and the language to describe them is different: they are called "dominant seventh chords" applied to any chord in the progression, and they appear in the ii-V-I patterns that chain together across keys. The chord progression of "All the Things You Are" by Kern is essentially a sequence of secondary dominants resolving through multiple keys.
SECONDARY DOMINANTS IN POPULAR MUSIC
Secondary dominants are not just a Classical phenomenon. They appear constantly in popular music, often where listeners do not consciously notice them but where they contribute to the song's harmonic interest. The Beatles used secondary dominants extensively. In "Something" (Harrison), the verse harmonization moves through a sequence of secondary dominants that creates the song's characteristic floating quality.
In jazz standards, secondary dominants appear in nearly every measure. "Autumn Leaves" begins with a ii-V-I in the relative major, then moves to a ii-V-i in the home minor — implicitly using the relative major's V chord as a tonicization. "Have You Met Miss Jones" by Rodgers and Hart contains a bridge with rapid modulation through secondary dominants. Learning to hear these moments in jazz standards is essential for any musician wanting to improvise or arrange in the idiom.
SECONDARY DIMINISHED CHORDS
Secondary dominants have a close cousin: secondary leading-tone chords. These are diminished chords (vii°7 or vii°) applied to scale degrees other than the tonic. The notation is vii°7/x. In C major, vii°7/V (the leading-tone seventh of the dominant) is F#-A-C-Eb, which resolves to G7 or G. The function is identical to a secondary dominant — to tonicize a non-tonic chord — but the chord quality is different: where V7/V is a dominant seventh, vii°7/V is a fully diminished seventh. The two are nearly interchangeable in function, and composers choose between them based on voice leading and harmonic colour.
DIATONIC VS CHROMATIC HEARING
The presence of chromatic notes is the primary signal of secondary dominants. In a strictly diatonic passage, all notes belong to the key. As soon as a non-diatonic note appears, you are encountering either chromaticism (passing tone, neighbor tone, modal mixture) or a secondary dominant. The follow-up resolution typically distinguishes these: if the chromatic note is the leading tone of a chord that immediately follows, you have a secondary dominant. If the chromatic note resolves back to a diatonic note without changing the harmony, it is a chromatic embellishment.
For ear training, this means: when you hear a foreign note, immediately listen to what follows. The resolution tells you the function.
VOICE LEADING IN SECONDARY DOMINANTS
Secondary dominants resolve by standard voice-leading rules: the leading tone of the tonicized chord (the third of the secondary dominant) resolves up by step; the seventh of the secondary dominant resolves down by step. In V7/V → V (D7 → G in C major), the F# (third of D7) resolves up to G; the C (seventh of D7) resolves down to B (third of G). The result is smooth, characteristic voice leading that defines the sound of tonal harmony.
This voice-leading consistency means that secondary dominants in dictation often have a predictable sound: a chord with a chromatic note that resolves to the chord whose root it is approaching. Once you internalize this pattern, secondary dominants become easy to identify in real musical contexts.
PRACTICAL EAR TRAINING EXERCISES
Start with V7/V in major keys. Play a I-V7/V-V-I progression on a piano (C-D7-G-C in C major). Listen to it dozens of times until the sound of V7/V is internalized. Then add V7/IV (C-C7-F-C, with C7 being the secondary dominant of F). Then V7/ii (C-A7-Dm-G7-C). Build up the secondary dominant vocabulary one chord at a time, with extensive listening at each stage. Aldwell and Schachter's textbook provides progressive examples through dozens of exercises.
SOURCES & FURTHER READING
Aldwell and Schachter. Harmony and Voice Leading. Schirmer/Cengage. Chapter 23: Tonicization. Laitz. The Complete Musician. Oxford University Press. Chapter 18: Secondary Dominant Chords. Open Music Theory — Secondary Dominants (openmusictheory.net, CC BY-SA 4.0). Benward and Saker. Music in Theory and Practice, Vol. II. McGraw-Hill. Chapter 12. teoria.com — Chord Identification Exercises including chromatic chords.