The circle of fifths gets its name from the fact that as you go from one section of the circle to the next, you are going up or down by an interval of a perfect fifth.

The keys that are most distant from C major, with six sharps or six flats, are on the opposite side of the circle. The next most closely related keys to C major would be G major (or E minor), with one sharp, and F major (or D minor), with only one flat.

This puts them in the same 'slice' of the circle. So the most closely related key to C major, for example, is A minor, since they have the same key signature (no sharps and no flats). What makes two keys 'closely related' is having similar key signatures. Keys are not considered closely related to each other if they are near each other in the chromatic scale (or on a keyboard).

In theory, one could continue around the circle adding flats or sharps (so that B major is also C flat major, with seven flats, E major is also F flat major, with 6 flats and a double flat, and so on), but in practice such key signatures are very rare.

The major key for each key signature is shown as a capital letter the minor key as a small letter.

D major D flat major