The Basics of Music Theory - Part 4 (Cycle Of 4ths)
Now that we have looked at key signatures (see The Basics of Music Theory - Part 3 (Key Signatures)), we are going to delve into the mysterious cycle of 4ths or 5ths depending on which way you see it. It is important and useful to be able to recognize each key and the notes that derive the key quickly. We often place the keys in an order. This order is called the cycle of 4ths or 5ths (some also use the term circle due to its diagram as seen below). It is called this because each key proceeds to the next key by the interval of a perfect 4th or 5th depending on the direction (ascending or descending) you are moving.
The Cycle Of 4ths Flat Keys
By looking at the chart below you can follow along with the following explanation. We begin with the key of C major which has not sharps or flats. It is placed at the top of the circle. Moving counter-clockwise the next key is F major. It is a perfect 4th from the key of C major. The notes in F major are F - G - A - Bb - C - D - E. The next key moves another perfect 4th to Bb major. Notice that F major has one flat and Bb major has 2 flats (Bb - C - D - Eb - F - G - A). The cycle continues by the interval of a perfect 4th around the circle until it reaches the very bottom.
The Cycle Of 4ths Sharp Keys
Reaching the bottom you will notice that there are enharmonic beginning at Db/C#. Moving two more perfect 4ths to Cb/B we cross over into the sharp keys. These keys contain sharps and no flats. Because the key of Cb would contain 7 flats, we use its enharmonic of B major to begin the sharp keys. This just makes it easier to work with and also gives an even number to our system. By continuing the movement counter-clockwise, the number of sharps will decrease until reaching the apex of C major.
The Cycle Of 5ths
You have probably noticed the two arrows at the top of the diagram. The left arrow indicates the movement in perfect 4ths and the right arrow indicates the movement in perfect 5ths. Both movements have their place in music theory but most popular music moves by perfect 4ths rather than perfect 5ths.
You might remember from our last discussion on key signatures that each major key has a relative minor. You The diagram indicates the major keys on the outside of the circle and the relative minors on the inside of the circle.
Next we will begin taking a look at chord construction and inversions. If you missed anything from before, then follow this link (The Basics of Music Theory - Part 3 (Key Signatures)) to the previous part of this series. Hopefully, this will be of value to you along with the upcoming exercises to really expand your knowledge of the guitar and music theory. If you are ready for more, then move on to The Basics of Music Theory - Part 5 (Triads and Inversions).
FYI: In case you are wondering about how I made the cycle of 4ths diagram, then look no further than Adobe Illustrator. Adobe is definitely the standard when it comes to vector graphics. As a matter of fact, almost all the images you see on this site have been touched by one of the Adobe products.