Tritone substitution is a method of replacing a Dom7 chord with another Dom7 chord that shares the same tritone, and it sounds terrific.

Diatonic chords are chords made using the notes within a scale. This makes them fit well together and makes your chord progressions coherent

Music is all about conveying emotions and stories through sounds. Every good story needs a conflict to make the resolution stronger. It's very hard to keep the audience interested in a song that has static emotion throughout. That is what "Dominant" means in music. The quality of a sound to have tension that has a strong pull towards another sound, i.e. the "tonic", to resolve the tension. This is the broadest understanding of the concept, but it can have several specific mea