Secondary Dominants

A secondary dominant is a dominant seventh chord that resolves to a chord other than the tonic. It is built exactly like V7, but it is aimed at a different target. Instead of pulling toward I, it pulls toward ii, IV, V, vi or iii.

In C major the only dominant seventh the scale gives us is G7 (G B D F), and it resolves to C. Its root, G, sits a perfect fifth above its target (or, the same thing, a perfect fourth below it). That fifth relationship is what makes a dominant feel like it *belongs* to its target.

Now aim the same relationship at another chord. To reach Am (the vi chord) with the same pull, build 1 3 5 b7 a fifth above A: that is E7 (E G# B D). We write it `V7/vi`, spoken "five-seven of six". The E7 briefly treats Am as if it were home. Musicians call this tonicizing Am, and the chord doing it is an applied dominant.

The most common secondary dominant is `V7/V`, the dominant of the dominant. In C major the dominant is G, a fifth above G is D, so V7/V is `D7` (D F# A C), and it resolves to G or G7.

The scale chord on D in C major is Dm (D F A). Raising its F to F# turns it into a major chord, and that F# is the leading tone of G: it sits a minor second below G and wants to rise into it. Add the b7 (C) and you have a chord with two tendency notes pulling toward G: F# up to G, C down to B.

Compare C-Dm-G-C with C-D7-G-C. The first is smooth and diatonic; the second leans forward into the G. Classical cadences use this constantly as I-V7/V-V7-I, and it is all over folk, country and pop as well.

`V7/ii` is the dominant of the ii chord. In C major the ii chord is Dm; a fifth above D is A, so V7/ii is `A7` (A C# E G), resolving to Dm.

The scale gives us Am on A. The secondary dominant raises its C to C#, and that C# leans up into D. Meanwhile the b7 of the chord, G, falls to F, the third of Dm. Two voices each moving a minor second into the target: that is the sound of a dominant doing its job.

This chord is the engine of one of the most played progressions in jazz. Put A7 in front of a ii-V-I and you get A7-Dm7-G7-Cmaj7: three fifths in a row, A → D → G → C, each chord the dominant of the next.

`V7/vi` is the dominant of the vi chord, the relative minor. In C major vi is Am; a fifth above A is E, so V7/vi is `E7` (E G# B D).

Here is the voice leading in detail. The third of E7, G#, is the leading tone of A and rises a minor second to A. The seventh, D, falls a step to C, the third of Am. The B can stay (it is the fifth of E7) and then move to C or E. If the target is Am7 (A C E G) instead, the D still falls to C, because Am7 has no D in it.

Why does this one sound so emotional? Because G# is the sharpest-sounding note you can add to C major without changing the key, and it lands on the darkest chord of the key. C-E7-Am is a favourite opening move in ballads and Latin music.

Two more targets remain: IV and iii. `V7/IV` in C major is `C7` (C E G Bb) resolving to F, and `V7/iii` is `B7` (B D# F# A) resolving to Em.

V7/IV is the one you hear in every blues. The blues I chord is already a seventh chord, C7 in C, and each time it moves to F7 it is behaving as V7/IV. Country and gospel love the same move: C-C7-F.

V7/iii is rare because Em is a weak target; when B7 does appear it usually keeps moving, B7-Em-A7-Dm-G7-C, as the start of a longer chain of fifths. Notice there is no V7/vii°: the vii° chord (Bdim) is unstable, so nothing tonicizes it.

C major has five secondary dominants, one for each chord you can tonicize. Each brings exactly the chromatic note (or notes) needed to make it a 1 3 5 b7 chord, and those are the only chromatic notes it brings.

Read the roots downward: A B C D E. The secondary dominants of C major sit on the scale degrees 6 7 1 2 3. And notice that every new note is the leading tone of its target (C# for D, F# for G, G# for A, D# for E), except in C7, where the new note Bb is the falling seventh, because C is already major and needs no raised third.

Minor keys have secondary dominants too, built by the same recipe: go up a fifth from the target and build 1 3 5 b7. Let's build them in A minor, whose chords are Am Bdim C Dm Em F G (i ii° III iv v VI VII).

Watch the labels: V7/VI is C7 → F and V7/VII is D7 → G. The chord `E7` is not on this list. In A minor E7 is simply the primary V7, taken from harmonic minor, resolving to i. A minor's own dominant is already chromatic (G#), so its secondary dominants sit on top of that.

The most striking one is `V7/III`: G7 → C inside A minor sounds like a sudden glimpse of the relative major, because it *is* the relative major's V7-I. Also notice that A7, C7, D7 and G7 are the same chords a C major piece would use; minor and its relative major share their secondary dominants, just with different labels.

A secondary dominant can itself be the target of another secondary dominant. String them together and every chord is the V7 of the next: a chain of dominants falling by fifths until it lands on the tonic.

Read it backwards from the tonic. G7 is V7 of C. D7 is V7/V, the dominant of G7. A7 is the dominant of D7, and E7 the dominant of A7. Each chord contains the leading tone of the next, so the whole line feels like it is tumbling downhill. Each resolution is only a half-way rest before the next push.

This is exactly the bridge of George Gershwin's "I Got Rhythm", the tune whose chords became the jazz "rhythm changes". In its home key of Bb the bridge is D7-G7-C7-F7, two bars each, landing on Bb: four dominants in a row, each the V7 of the one after it. Play the chain in C below and you will hear the same shape.

The quickest way to spot a secondary dominant is this: a major (or dominant seventh) chord where the key would give you a minor one. In C major, seeing E or E7 where you expected Em is the giveaway.

Three confirmations. First, the chromatic note is a raised note acting as a leading tone (G# in C major means A is coming). Second, the root falls a fifth into the next chord. Third, the tonic does not change. A bar later you are clearly still in C. Even the plain triad works: a secondary dominant does not need its seventh to do its job, E → Am pulls almost as hard as E7 → Am.

A tritone substitute is a dominant seventh chord that replaces another dominant seventh whose root is a tritone away: an augmented fourth, exactly half an octave. In C major, G7 can be replaced by `Db7`, and it still resolves to C.

Why does it work? The tension in G7 lives in its third and seventh, B and F, a tritone apart. Db7 has the very same two notes: F is its third and Cb (= B) is its seventh. Only the roles are swapped. When G7 resolves, B rises to C and F falls to E; when Db7 resolves, exactly the same B → C and F → E happen. The pull is preserved.

What changes is the bass. Instead of leaping a fifth (G to C) it slides down a minor second (Db to C). In a ii-V-I, replacing G7 with Db7 gives the bass line D-Db-C: the smooth chromatic descent that is a signature of jazz and bossa nova. The substitute is written `subV7` (or bII7).

Any dominant seventh can take a tritone substitute, including secondary dominants. Move the root of a secondary dominant by a tritone and you get a chord that sits a minor second above its target and slides down into it.

Put several of these in a row and the bass walks down the chromatic scale. Cmaj7-Bb7-Am7-Ab7-Gm7-Gb7-Fmaj7: the bass goes C Bb A Ab G Gb F, one chromatic step at a time, and every dominant is a tritone substitute aimed at the chord after it. This is the sound of bebop turnarounds and countless film-score endings.

Everything in this chapter has been tonicization: a chord is treated as a tonic for a moment, then the real key takes over again. That is different from modulation, where the music actually moves to a new key and stays there.

Secondary dominants are the bridge between the two. Every chromatic note they bring, F#, C#, G#, Bb or D#, is a note from a neighbouring key, visited for a beat and then released. Chain enough of them and you are on the way to a modulation; use one and come straight back, and the key never wobbles.

In the next chapter, Other Scales, we leave the seven-note world of major and minor for pentatonic, blues and symmetrical scales. Later, in Modal Interchange, we meet the *other* way to bring chromatic chords into a key: not applying a dominant, but borrowing a chord from the parallel minor: bVII, bVI, iv. Applied dominants pull; borrowed chords colour. You will want both.