One VERY common substitution is called a "tritone substitution" or more commonly a "tritone sub." Although it is usually used as a transition chord, in which you have two means of resolution, it can also be used as a complete subsitution giving some very nice tension. In order to substitute for a tritone sub, simply take a dominant 7th chord, and transpose up to a dominant 7th chord a tritone (diminished 5th/augmented 4th) away from the root. Tritone subs are also a great way to get away from the overused II-V-I progression because you can substitute the V chord for a tritone sub of the V.

For an example of the substituting the diatonic II-V-I progression with a substituted ii-IIb-I progression try this:

Here's the original - Dm7 / G7 / CM7
Here's the substituted - Dm7 / Db7 / CM7

You could also just straight up substitute a chord for it's tritone sub as well, while keeping the same bass line for an interesting effect. If you'd like to know how to resolve a tritone sub, classical style, let me know...but it's best just to "sound it out."

Hope this helps...if you want more examples, let me know.

Mint. I heard something like this. Gonna try it. Thanks.

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^^ that's why Neo is helping me with the theory tutorial

that is.. once I get around to it

I have 11 pages due for classes next week.. and then I get to worry about finals.. I'll try to fit it in within the next week or so..

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I assumed it was going to be in the tutorial. I believe Kewlness volunteered to do an advanced tutorial if you and Neo are too busy to delved too deep into theory.

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