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In the Reddit FAQ question How are 7th Chords Used?, the following explanation appears:

Dominant 7th chords are also useful in modulation as secondary dominants - the addition of the seventh will create a much stronger pull to the tonic chord you are aiming for

What does this mean? What would be an example in C major modulating to G major or E minor?

See also What is a secondary dominant chord?

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We're in the key of C. Then comes a secondary dominant chord to get us to A minor.That chord will be E, or E dominant 7. The reason it works well is resolution. Notes from one chord moving as little as possible to the next. The A minor has A, C and E. The secondary dominant (in this case, although it is the dominant of Am) has E, G#, B and D. The E note stays constant, the G#, being a leading tone, pushes a semitone up to the A. B has a semitone to get to C, and D could be thought to move a tone each way to arrive at C or E.

Your specific in C, going to G, would involve the dominant of G, D7. To modulate to E minor, the secondary dominant will be B7 : B stays, D# up a semione to E, F# up a semitone to G, and A (the b7) goes a tone either way to G or B.

Interesting that the major 3 and the minor (b) 7 of the secondary dominants form a tritone, which tends to give an unstable feel to the chord.

  • Thank you Tim for writing an answer. For some reason the site didn't send me a notification until yesterday :-/ – Sonia Hamilton Dec 4 '16 at 23:08
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A dominant chord is a major chord built on the fifth degree of a scale. It has a strong tendency to resolve to (go to) the tonic chord fifth below. For example in C major, G major is the dominant chord and it "pulls" towards the tonic C chord. In G7 chord, this tendency is even stronger.

A secondary dominant is the dominant of a chord other than the tonic chord. Due to its strong tendency to resolve to a fifth below, it is often used to modulate to different keys.

For example, D7 is the dominant of the G chord so it can be used to modulate to G major (or minor for that matter) from more or less any other key. Similarly a B7 chord can be used to modulate to E minor.

  • Or, less jarringly, A7 helps modulate to D. A7 has two notes (E,G) in common with C, along with one sharpened note (C# as opposed to C). – No'am Newman Nov 16 '16 at 8:36
  • Thank you cyco130 for writing an answer. For some reason the site didn't send me a notification until yesterday :-/ – Sonia Hamilton Dec 4 '16 at 23:08
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If you wish to establish a new tonal centre, a good method is to precede it with its dominant chord. Strengthen it even further by preceding the dominant seventh with ITS dominant. This is referred to as a string of secondary dominants.

Example. In C major.

C, Am7, Dm7, G. Probably no modulation. The next chord is likely to be C.

C, Am7, D7, G. Maybe a modulation. G is preceded by its dominant 7th chord. The music could proceed with G being treated as tonic.

C, A7, D7, G. More chance again of it being a modulation, rather than 'just visiting'.

In all cases, what happens next is more important than the chords leading up to the G chord. OK, we've got there. But are we just passing through, are we pitching a tent, are we building a house.. ? Is a passing visit a 'Modulation'? Maybe. It's only a word. Between 'just passing through' and 'settling down for a long stay' are many degrees.

  • Thank you Laurence for writing an answer. For some reason the site didn't send me a notification until yesterday :-/ – Sonia Hamilton Dec 4 '16 at 23:09

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