What is a secondary dominant?

A secondary dominant is a chord that is not part of the key in which it appears, but rather belongs to the key of a chord that follows it. It is usually a major chord built on the fifth scale degree of the chord it resolves to.

For example, in the key of C major, the V7/V chord would be D7, because D is the fifth scale degree of G major, which is the dominant chord in the key of C. The D7 chord creates tension and leads naturally to the G major chord that follows it.

Secondary dominants are commonly used in jazz, musical theater, and other genres to add complexity and interest to chord progressions. They can also be used to modulate to different keys or add a sense of movement and flow to a song.