Welcome to Music Modulation.

Tonality does not exist in absolute terms, it manifests itself through a harmonic movement where the chords have relationships with each other. Relationships of tension or relaxation around a central chord called Tonic. A Tonality can be defined by the key signature. 
Moving from one key (Tonality) to another one is called, Modulation.

  1. Direct Modulation. (moving directly from one key to another)
  2. Using transition chords (V7), "dominant 7th chords" See Chords lll page.
  3. Using Pivot chords and Pivot notes. (yet to come)

Direct Modulation going from C to G using F# as a leading note of G Major tonality.
Matteo Carcassi - Op.60.pdf