E.g major scale is I ii(m) iii(m) IV V vi(m) vii(dim). I also know chord formulas and harmonies for minor (natural, harmonic and melodic) scales.
I think I understand the usage of V of V. It is a borrowed chord from other scale, and since it is a dominant chord of 5th of a scale it will fit in, right?
But I'd like to understand the theory better here, and where does it come from (classical, jazz, blues...). To be able to apply V of V to music.