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If we apply that to a simple scale (C major is pretty easy) it would look like:
I | major 7th II | minor 7th III | minor 7th IV | major 7th V | dominate 7th VI | minor 7th VII | minor 7th flat 5
Now you say to yourself, "Self, how does that help me?" Well lets say you saw a chord progression with a D-7, G7, Cmaj7. You say, "Well, what scale would have a D-7, a G7, and a Cmaj7? Why a C major scale, of course!" Easy stuff, right? Try one on your own. Say you saw a A-7, and a D7. Well, a few scales have a A-7, but only one has a A-7 and a D7. If you guessed G major, you are right. this brings me into the next topic, jazz harmony. The most common chord progression you will ever see in jazz is a II V progression. That is very important so I will tell you a few more times. The most common chord progression you will ever see in jazz is a II V progression. What? I forgot what you said. The most common chord progression you will ever see in jazz is a II V progression. Oh thanks. I'll be sure to remember that now. What does this mean? It means that whenever you see a minor 7th chord followed by a dominate 7th chord that is a perfect 4th above it (2 to a 5 is a 4th) chances are that it is a II V progression so the key you will be in is whatever key has those notes for a 2 and 5. Now we will talk about switching keys. Cool! How do we know when we switch keys? Well when we see chords that shouldn't be in the key we're a; ready in. So if we are seeing D-7 G7 D-7 G7 and all of a sudden we see a Eb-7b5, we know something is up. What is up? A key change, of course. That's basically how you know, or also if you see another II V progression thing that's not in your old key, that also means that you have to switch keys. So when you switch keys, you have to do the whole finding out what key your in thing over again. Ok thats about it for finding what key your in now we learn what scales to use. 1st, we have to learn about "modes." Lets assume we're in C major, just to make it easy (so what chords will we be seeing?) You should know how to play a C major scale. Who doesn't? And if you don't, learn fastbecause you won't get much farther without knowing that. So we know, the notes in a C major scale are: C D E F G A B C. Right? Right. Now, what would happen if we played those same notes, but started from the D? We would have: D E F G A B C D. This mode is called Dorian. Know that. Now lets start from the E: E F G A B C D E. That mode is called Phrygian. Know that. Now lets start from the F: F G A B C D E F. That mode is called Lydian (are you writing these down?) and so on. Here's a brief list of all the modes. The roman numeral represents the scale degree you start from, and the name is supplied after.
I | CMaj7 II | Dmin7 (D-7) III | E-7 IV | FMaj7 V | G7 ("tritone" - dominate) VI | A-7 VII | B-7b5
Figure out how to play these on a guitar. go all two octives, be able to arpeggiate them and do all sorts of crazy things with them. know them like the back of your hand. Now you say to yourself, "Self, how do I know which scales to play? Oh, wait, thats what this lesson is about. Maybe I should read on." Well, to tell you the truth, it is not especially critical what scales you do play, just so long as they're in the right key. Remember, a Dorian, Phrygian, etc. scale is in it's respective major key. I.e. a G Mixolydian is in the key of C major. Most of the time, you; ll have to play it by ear. Literally. There are ways to know exactly what to play and when but that requires a few years of college and I'm just a 14 year old guitar player who is typing what I know. Basically, in a II V progression, you will want to be playing either the respective Dorian or Mixolydian scale. Apreggiating sounds pretty good, so do chormatic guide-tones (guide-tones coming up soon). Remember for jazz never ever do a bend. It just sounds like crap. Don't ask be why, it just doens't fit. Remember in english class when they tell you that your writing needs to have transitions in it? "On the other hand" "Conversly" "However" "Unlike..." Well, that's what guide-tones (GTs) are like. GTs are transitions between keys in music. Now I will tell you a little bit about how to use them. Two guide tones that you will see a lot 3rds and 7ths. The reason that these are common is that it is the 7th and the 3rd that really define the chord. They make it major, minor, dominate, so if you hear a major 3rd, you will right away be thinking: wow! Major! Not minor! It is important to find notes that are common to both keys. The key that you are in and the one that you are changing into. It just makes it smoother. Other GTs include doing chromatic (half step) assencions or dessencions into notes in the new key. That's about it. I know I've really only touched on the surface but its the best I could do in half an hour.
I | Ionian (aka. major) II | Dorian III | Phrygian IV | Lydian V | Mixolydian VI | Aeolean (aka. minor) VII | Locrian