Watch the The Cycle of 4ths (and 5ths) online guitar lesson by Frank Vignola from Modern Method for Guitar
The cycle of fourths (also called the cycle of fifths) shows the relationships among the twelve notes and tones of the chromatic scale, their corresponding key signatures and related major and minor keys. Refer to the diagram of the cycle of fourths. If you count letter names counter clockwise from C you see that the next key, F, is 4 notes away. Counting clockwise from C the next key, G, is 5 notes away, hence the name cycle of fourths or cycle of fifths.
In the lessons that follow, we will often practice our exercises around the cycle of fourths, C-F-Bb-Eb-Ab-Db-Gb-B-E-A-D-G, rather than the chromatic scale. Using the notes that make up the cycle of fourths as a starting point is a more musical approach because it emulates chord changes you find in a piece of music.