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Music theory can be complex, especially when it comes to understanding how chords function within a progression. Two common concepts that often confuse students are tritone substitution and secondary dominants. Although they both involve altering or emphasizing chords, they serve different purposes and operate under different principles.
What Are Secondary Dominants?
Secondary dominants are chords that temporarily tonicize a chord other than the tonic in a key. They are essentially “borrowed” dominant chords that lead into a specific target chord, making it sound more prominent or “highlighted” within the progression.
For example, in the key of C major, the V of the ii chord (D minor) is A7. Playing A7 before D minor creates a strong pull towards the ii chord, emphasizing it within the progression. This is called a secondary dominant because it functions as the V chord of the target chord.
What Is Tritone Substitution?
Tritone substitution involves replacing a dominant seventh chord with another dominant seventh chord a tritone away. This substitution creates smoother voice leading and adds harmonic interest to progressions.
For example, instead of G7 resolving to C, you might use Db7 (a tritone away from G7) to resolve to C. The notes in G7 and Db7 share a tritone interval, which makes the substitution sound natural and often more colorful.
Key Differences
- Function: Secondary dominants emphasize a specific chord, making it sound more important, while tritone substitutions replace a dominant chord to create smoother or more interesting progressions.
- Location in Progression: Secondary dominants usually appear directly before their target chords, whereas tritone substitutions can be used more flexibly to replace dominant chords anywhere in the progression.
- Interval Relationship: Secondary dominants are based on the V of a chord, whereas tritone substitutions are based on the tritone interval (three whole steps apart).
- Harmonic Color: Tritone substitutions often add a jazz or blues flavor, while secondary dominants tend to create a more classical or functional harmony.
Summary
Understanding the differences between tritone substitution and secondary dominants helps musicians craft more compelling harmonic progressions. Secondary dominants highlight specific chords, adding emphasis, while tritone substitutions offer a way to add harmonic variety and smooth voice leading. Both tools are essential for advanced harmonic development and improvisation.