Secondary dominant
A secondary dominant chord is a non-diatonic dominant chord. It is useful when you want to make movement towards a chord in a progression.
A simple progression like:
G | G | Am | D
could be changed to:
G | G | Am A7 | D.
A7 is the V7 chord of D.
The roman numeral notation is “V/X” where X is the target chord. In the above case, the roman numeral analysis would be I | I | IIm V7/V | V.
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