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I'm studying harmony and start to make roman numeral analysis on Bach pieces.

I often mistake a II (second degree) for a secondary dominant on the 2nd degree(V7 of V).

I guess there are some notes differences :

  • the minor 7th must be present in the dominant seventh to create a tritone while the II can just be a triad.
  • in a major context, II will be minor (ex : Dm in C) while V7 of V will have a major third (ex : D-F#-A-C in C)...

My questions are :

  • Are these the only difference in notes?

  • Apart from the notes, what is the difference in terms of harmonic roles between II and V of V in a chord progression?

Thanks.

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2 Answers 2

up vote 7 down vote accepted

First let me make this remark: as always when analyzing, know what key you are in and look for accidentals outside the key. If there are no accidentals outside the key then you can't be dealing with a secondary dominant.

Now let's look at the chords in the key of C major:

ii:   D  F  A
V/V:  D  F# A

ii7:  D  F  A  C
V7/V: D  F# A  C

As you can see the only difference is the F# in both the triad and the seventh, but that is a BIG difference because F# is not in the key of C so that accidental should be a big red flag that you are going outside the key.

As for function it is very simple. They both go to V, but they do it very different ways. ii and ii7 are both predominants so they want to go to a dominant chord (either V, V7, vii°, or vii°7). V/V and V7/V are both dominants that tonicize V (make V sound like the tonic).

The big difference is again going back to the example above is the F# that acts as the leading tone when used as a secondary dominant and makes G feel like tonic. You don't have that tonicization when you are just using ii or ii7.

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You can see a good example of the use of secondary dominants (such as V/V) in the hymn "Now Thank We All Our God" (in the key of Eb, in the music I'm looking at). The word "our" in "hath blessed us on our way" has a V/V. Note the A natural, the leading tone for Bb. This is part of F-A-C, the V of Bb which is the V of Eb. This strengthens the "fiveness" of the chord witout actually moving to the new key. If you go further, to the "of" in "with countless gifts of love", you will see a V/ii. (Again, note the E natural.) This V/ii strengthens the "twoness" of the ii chord. –  BobRodes Mar 5 at 15:00
    
Thank you, you made me discover the concept of "Tonicization" (treatment of a pitch other than the overall tonic as a temporary tonic). –  kurto Mar 6 at 10:14

The ii of a major key is a triad (in C) D-F-A. adding a minor 7th will make it minor 7th ( !!) -D-F-A-C. This usually resolves to C through the V - G, or G7.

When a piece may modulate to its dominant (now we're in G ), that ii usually becomes a II, as in D - F# - A (with the optional 7th of C ).That produces the tritone tension to take the piece unequivocally into G.The F# is the leading note of the new key, G.

Using a Dmin. will not have the same push as using D maj. and putting the min.7th of that in as well gives the extra push.

ii-V-I (and II-V-I ) are very common changes , used lots in jazz. Bach used them occasionally.

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