Chromaticism

The use of accidental notes outside the prevailing diatonic key either for special effects (emotionally intense parts; colour-modification of diatonicism) or for modulation. The augmented sixths (augmented triad=affecting chord), the Neapolitan sixth and the supertonic chromatic chords are examples. The chromatic or diatonic character of a ‘foreign’ note is conditioned by harmonic considerations like: in the progression C-E-F#-G, the note F#  is chromatic if the harmony stays on C, diatonic if it modulates to G.