The Circle of Fifths
 is a way to arrange keys to show how closely they are related to each other.  The major key for each key signature is shown as a capital letter and the minor as a small letter.  Keys are not considered closely related because they are near each other in the chromatic scale or on a keyboard, they are closely related because they have the same key signature.  So the most closely related key to C major is a minor because they share the same key signature.  The next most
closely related key would be G major (or E minor) with one sharp or F major (or D minor) with one flat.  The circle of fifths gets its name because if you start with C and go clockwise each key is a perfect fifth up from the last.  If you start with C and go counterclockwise each key is a perfect fifth below the last.


 

Circle of fifths