Secondary Dominants:Major triads OR Major-minor Seventh chords (Dominant Sevenths) that belong to the key of a diatonic chord.

V / iiRead: “Five OF Two”

Key of C: dofVdofA

A C# E

Secondary Progressions:

The leading tone chord also sounds dominant and can have the same function.

Secondary Progressions:

They do not necessarily establish a new key.

Dominant Expansion

Dominants in Sequence

They can resolve to a different chord to preserve voice leading principals.