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Any major or minor chord can be approached by its own temporary dominant or dominant 7th chord, which is called a “secondary dominant”.

  • Secondary dominants are major triads.
  • Secondary dominant 7ths are major triads with a minor 7th from the root added on top.
  • All secondary dominants are chromatic chords – they contain notes which do not belong to the current key.

The most frequently seen secondary dominant is the dominant of chord V.

In C major, chord V is G major. In G major, chord V is D major, so D major is “V of V” or “the dominant of the dominant”. In C major, the diatonic chord built on D is D minor, but because dominant chords are always major, “V of V” will be a chord of D major. The secondary dominant 7th chord is D7 (D-F#-A-C).

C major secondary dominants

A secondary dominant chord can be written as a single Roman numeral (e.g. II, or II7 for a supertonic chord), or it can be written as V/chord, for example V/V means “dominant of chord V”.

NB: Chord ii7 is a “secondary 7th”. Chord II7 is a “secondary dominant 7th”.

Here is a chord progression in C major, with a typical ii-V-I ending.

chord progression C major

Instead of using chord ii (diatonic), an alternative is to use chord II or II7 (chromatic).

chord progression with II7

The other chords here can also be preceded by their own secondary dominants.

In this version, chord IV is preceded by its own dominant 7th (I7 or C7), instead of chord iii.

I7-IV

Here, chord ii is preceded by its dominant (VI7 or A7), instead of chord IV.

VI7-ii

Finally, this version begins on the secondary dominant of vi (III7 or E7) and includes two other secondary dominants.

secondary dominant progression

Some additional points:

  • When moving into and away from a secondary dominant, each voice should move as little as possible – either moving to the nearest note, or repeating the same note if it exists in both chords.
  • Secondary dominants add a lot of colour and interest to a chord progression, but they can also feel unsettled, especially if several are used in close proximity.
  • Secondary dominants sound like “mini modulations” but they are not true modulations. The key changes for just two beats, and the overall key of the phrase does not change (the above example is in C major from start to finish).
  • In some countries, the process of using a secondary dominant is called “tonicization”, because the chord that follows the secondary dominant sounds like a temporary tonic. This term is not commonly used in the UK.

To determine whether a chord is “V7 of a new key” or “V7 as a secondary dominant”, look at the chords which follow this progression. If the subsequent chords match the key of the V7 chord, then it was a modulation.

In this example from Mozart’s Piano Sonata in A minor, the C# and E create a chord which does not belong to A minor. The second beat (bar 6) is a chord of Dm7, so it is possible that the C#/E belong to V of D minor, but has the music modulated to D minor, or is this a secondary dominant used in A minor?

secondary dominant, or modulation?

In D minor, Dm7 is chord i7, which is not a standard chord [1] in D minor, because the 7th of the scale is usually raised to C#.

The next chord is G major, which is also non-standard in D minor (normally uses Bb).

This is followed by B/D#, which is definitely not a chord in D minor.

There is no evidence that a modulation to D minor has taken place, so the C#/E should be considered as a secondary dominant moving to chord iv7 in A minor (V/iv7).

(The B/D# is another secondary dominant, but this one moves to its submediant (VI7 or Cmaj7), rather than its tonic Em.)

[1] apart from in jazz/pop etc.