In simpler forms of music, dominant 7ths most frequently occur as chord V7 of a major or minor key. However, it is possible to substitute a dominant 7th chord for any scale tone chord (e.g. II7, III7, or VI7. These are referred to as secondary dominants. They are useful for implying a series of V I progressions in different keys, while harmonizing a melody which stays within one key. Secondary dominants can be used in any style of music, and are particularly common in Jazz and Blues.
This example shows secondary dominants applied to a turnaround progression. Altered dominants can sometimes be used instead of a simple 7th chord.
In this example, secondary dominants are used to suggest a series of V I movements in various keys leading back to the starting point of C major.