So now you know that you can play many different chord progressions using just one major scale, right? And you know that using guide tones will help you sound even better. But did you know that most tunes don't stay in just one key? Well now you do! The trick is in recognizing these temporary tonal centers, and they are usually not denoted by key signature changes. As you work with ii-V-I's and other common chord progressions, you will be able to see this and to hear it. A good first trick is to look for the I chord and then work backwards from there to see if there is a ii-V leading there. That will tell you that all of those chords are in the same key. As an example, I will take the ii-V-I we have already done, in the key of Bb concert, and sneak another one in there somewhere. See if you can find it. Here it the progression.
There it is! The shaded area is a ii-V-I progression in Eb. So when you play this, for those 4 bars, you are going to want to play in concert Eb. These two keys are very close relatives.. brother and sister maybe, because their keys are very close. Bb has 2 flats, Eb has three, so the keys are very similar. All you have to do is change your scale slightly to make this change. But that scale change is very important. If you don't make it, the changes go away, and Jo Jones will throw a cymbal on the ground in front of you (obscure Charlie Parker reference, watch the movie "Bird" to understand it)
So, play the chord progression, listen to it, and then play the guide tones. After you are comfortable with that, play in the two appropriate keys.
So that's the nature of making the changes right there. Figuring out what scale you need to use for certain chords, emphasizing the guide tones, and soon we'll learn about arpeggiating the chords and using patterns. It's all vocabulary. You are learning to speak jazzish. But remember this, just in speaking words, it's not about using the biggest and most impressive words, it's about saying someing worthwhile. It's about telling a story, just as you would when you speak. So after you practice your vocabulary, just play and forget it. Don't think about chords, don't think about scales. Just play. Tell your story. Learn something so well that you don't have to think about it anymore.
Next we are going to play a little tune based on 3 ii-V-I progressions. Two you already know. This adds one more.