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?