Tritones
A tritone substritution is when you substitute a dominant 7 chord for another that is a tritone away. An example being you can use Db7 instead of G7 as Db is a tritone away from G.
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Learning Tritones and adding simple major chords over the top of them creates what called altered dominants. These are dominant seven chords with raised Fifth and or Ninth intervals. Enjoy this lesson from pianodownloads.com.