To change C Mixolydian mode into C Dorian mode, you remove the E and replace it with E-flat. (In this diagram and all the ones that follow, blue notes are flat or minor intervals above the root.)

To change the C major scale into C Mixolydian mode, you remove the B at the clockwise end of the necklace and replace it with B-flat at the counterclockwise end. For example, to change the C major scale into C Lydian mode, you remove the F, the furthest counterclockwise end of the necklace, and replace it with F-sharp, adding it onto the furthest clockwise end of the necklace. It’s easy to transform the major scale into its parallel modes by adding and removing notes from the ends of the necklace. This kind of circular representation is called a scale necklace (or sometimes scale bracelet). The green notes form major or “natural” intervals above the root.

The purple notes are the ones that form “perfect” intervals above the root C: unison, octave, fourth and fifth. Here’s the C major scale on the circle of fifths: But it also helps explain how the major scale and diatonic modes relate to each other, and gives a possible explanation for why they sound good. For one thing, it can help you understand how key signatures work.

If you want to understand Western music theory, the circle of fifths is an invaluable tool.

Circle of 5ths for a flat major