Secondary dominant

When a composer of music writes in a key then uses the dominant of a chord within that key, this is the use of a secondary dominant. This is one of the easiest and most facile ways of modulation. Example:: C Major: I--V--V/vi--vi--IV--V--I

                  C--G--E---a min--F--G--C

This example shows the E Major chord that stands as the secondary dominant of a minor--V of vi (the major V) going to vi (minor).