If you follow the key signatures carefully, you'll see that from C to G is a fifth, G to D is a fifth, D to A is a fifth, and so on through the sharp keys. Similarly, C to F is a fourth, and from F to B is a fourth, and so on through the flat keys. Essentially, the pattern is a fifth in one direction, a fourth in the other. Called the circle of fifths, this pattern is illustrated in FIGURE 6-6.

The circle of fifths is an easy but important way to learn the key of a piece of music because it tells you how many sharps or flats are in the key. C has no sharps or flats. The circle of fifths shows how when you go clockwise around the circle you go up a fifth. For example, the fifth note of the C major scale is G. The fifth note of the G major scale is D, and so on. Also notice how there are twelve notes on the circle of fifths, corresponding to twelve numbers on a clock.

How is this information helpful? Take any chord progression, for example, C major to F major. Now, suppose you'd switch that to A. In other words, you want the same exact chord progression, but you want it in A instead of C. Since F is in the position that is one turn counterclockwise of C, then all you have to do is go to A, and then go one turn counter clockwise to D. That is how easy it is to use the circle of fifths. (In guitar fretboard theory, the circle of fifths becomes easier to see when you start playing barre chords, described on page 133.)