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. The use of secondary dominants is 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. Here is a progression using secondary dominant chords.
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.