When referring to important factors such as "The Key of C", the progression must pertain from the Key in reference. The Dominant note in the Key of C Major is G and the sub-dominant note is F. If the tonality of a piece emphasizes to the dominant key, the sub-dominant note of the tonic has been sharpened (F to F#) which now becomes the lead tone in the dominant key (G). This sharpened 4th of the tonic would reference to C Lydian. This example would be one way to modulate, or change keys using a parallel Major in the Key of C and modulate to the Key of G.