Welcome to Music Modulation.
Tonality does not exist in absolute terms, it manifests itself through a harmonic motion where the chords have relationships with each other. Relationships of tension or release around a central chord called Tonic. On the other hands, tonality can be defined by the key signature. Moving from one key (Tonality) to another one is called, Modulation. There are several types of modulation.
- Direct Modulation. (moving directly from one key to another).
- Using transition chords (V7), "dominant 7th chords" See Chords lll page.
- Secondary dominant chords modulation.
- Using Pivot chords and Pivot notes. (yet to come)
3. Modulating with secondary dominant chords.
The definition of the secondary or borrowed dominant chords is the use of a chord that is foreign to the original key (Tonic). The excerpt below is from the Beatles song " In my Life". The song is in the key of A major.
So that to analyse those four bars of the song, I suggest we simplify it and just write the main chords of that section.
Now it looks easier to see what's going on here. We see immediately that A7 doesn't belong to the key of A and plays here the role of the foreign chord. So, in this case, to which key A7 belongs to? A7 is the dominant seventh of D key and the arrow shows us that effectively it resolves on D. Since we have started in the key of A this passage ends also in the same key, we have to analyse the all section in the key of A. This is why A7 is labelled (V7/IV) and D is noted as the subdominant chord in A key, namely the fourth degree (IV). To finish off the analyse, we have to mention the plagal cadence, D to A (IV) to (I) in the bracket.