Chords Based on the Major Scale

• If you substitute the numbers one through seven for the major scale, it is possible to consider any key. Chords are constructed by taking every other note of the scale. For secondary dominants, add the seventh degree to each diatonic chord: 1-3-5-7, 2-4-6-1, 3-5-7-2 and so on.
  
  

Dominant Chords

• Dominant chords are the chords that have the easiest and most direct resolution to the tonic (I chord). The dominant chord is always based on the fifth degree of the major scale, and is written as 5-7-2-4. In the key of C, for example, the dominant chord is G7.
  
  

Secondary Dominants

• These are dominant chords that lead to any chord but the tonic. They are expressed as the "five of" the chord where they are resolving. In the key of C, for example, an E7 chord is expressed as the V7 of vi (V7 of A minor).
  
  

Benefits

• Dominant-to-tonic (V-to-I) resolutions are the most fluid and common resolutions in western music. The secondary dominant allows for the same tension and release of V-to-I resolutions for any chord of the tonic key. The applications of secondary dominants create endless harmonic and sonic possibilities.