ii vs II and iii vs III

In the legend you may notice that the 'ii' and 'iii' chord have listed with them a 'II' and 'III.' This is an indication that you may notice songs that use major chords (II and III) when playing chords based on the 'second' and 'third' notes of the scale instead of the typical minor 'ii' and 'iii.' In the case of the key of C, this would be using D Major instead of a D minor ('II' instead of a 'ii') and using a E (Major) instead of a Em ('III' instead of a 'iii'). Try using your Chord Wheel to analyze the songs of Jimi Hendrix. You'll find he had quite a penchant for 'III chords' while Elton John and Bob Dylan uniformly stick to 'iii chords.' The Beatles on the other hand, loved to use 'II7 Chords' and even 'III7 Chords.' Refer to the 'Analyzing Compositional Styles' section.

Like many of the concepts behind the Chord Wheel, there is an infinitely more complex chord theory explanation behind this 'II' and 'III' substitution. It has to do with phrases like 'Secondary Dominants' and should you wish to further your comprehension of music theory via additional research, it will be explained in agonizing detail. However, the entire purpose of the Chord Wheel is to have you enjoying the essential, practical benefits of chord theory without exhaustive study.

On the Chord Wheel's clear disc, you'll see the 'II Chord' and the 'III Chord' are also outlined above their respective roots. Thus when in the key of C, the D is outlined just to the right of the G. This is an indication that D (or as it prompts you, a D7) will often be found in 'Key of C' progressions.