In lesson 38 You learned that 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 demonstrating the use of secondary dominant chords.