Let's keep it simple here .. We're going to tackle a single subject called chord substitutions. The concept is simple - a chord substitution substitutes one chord for another! Okay, let's substitute Ama7 for Dbmi7b5. Well, those two chords don't substitute very well for each other. Why! Because in a good chord substitution, the two chords have at least one and usually more notes in common. If two chords have all their notes in common, then they substitute very well for each other because they're the same chord!

So let's look at a very particular kind of chord substitution common in the jazz world called a flat five substitution. Two dominant 7th chords whose roots are a diminished fifth apart (aka, a flat five) can substitute for each other quite nicely. Why? Because they have two of their four notes in common! Let's check it out ...In our example above, we see a Db7b5 chord. Forget about the b5 part. It's some kind of a D dominant seventh. It's own dominant seventh chord is an A7, right? But instead, we see an Eb7 moving towards the D chord. If we look closer, we see that A7 and Eb7 are a diminished fifth apart. The notes in an A7 chord are A, C#, E, and G; the notes in an Eb7 chord are Eb, G, Bb, and Db (or C#). These two chords have G and C# (or Db) in common! So they can substitute very well for each other!(These two notes are also a diminshed fifth apart, and so generate a very colorful interval on their own, but more on that some other time.)

So why substitute one chord for another when the first one will quite nicely? Another excellent question. Here are some reasons:

1) It can introduce an element of surprise into the chord movement while still having parts of the original progression..

2) It can open up new avenues for modulation, which is good if you want to go somewhere else..

3) Because it is a very musical thing to do ...

4) Because it's cool and your friends will want to know how you did it ..

So why would you not substitute one chord for another? And yet another excellent question. Here is one really good reason:

1) Because it doesn't fit the style of music you're working in .. a really good way to break up a bluegrass jam? Play flatfives subs, as many as you can find ... !