Secondary dominant

Secondary dominant (also applied dominant, artificial dominant, or borrowed dominant) is an analytical label for a specific harmonic device that is prevalent in the tonal idiom of Western music beginning in the common practice period. In the tonal idiom, a song or piece of music has a tonic note and chord, which is based on the root of the key that the piece is in. The most important chords in a tonal song or piece are the tonic chord (labeled as I in harmonic analysis) and the dominant chord (V). In the key of C major, the tonic chord is C major and the dominant chord is G.