After I have explained all of this, you're probably still wondering (if you're like one of my students) how to play in the key. I wish I had a dollar for every guitar player I've met that knew how to play some scales but didn't know how to apply them. Whatever is said here can only be understood as a set of general rule.

It's a bit like the time I watched a baseball game with a guy from Liverpool, England who had never seen a game. He asked some basic questions, such as, "How long does the starting pitcher pitch?" Customarily, it is that a pitcher goes for about seven innings, then the reliever is brought in. Yet, a few years ago, the Indians went through all of their pitchers in one inning. So as you can see, there are some general statements that are usually true about the game, but not always the fact. The same is true about playing in the key - but the moment I make some sort of rule, you will discover an exception to that rule. So, USUALLY, when you start and end on a chord, that's the key you are in.

Most of the music we listen to (unless you're into John Cage, avant-garde, college radio or industrial noise) has a tone center. The music and chords start out in a certain key, go through a small musical journey (called a cadence) and return from whence they came to the beginning of the chord. Understanding that it sounds simple, it is nonetheless true.