If you superimpose the major circle with the minor circle, you can see that each major key shares a key signature with a minor key. These two keys are considered relative to each other. The major key is always three half steps higher than its relative minor key. If you were asked to provide the relative minor key for F major, all you would have to do is count three half steps down from F to arrive at d minor. Piece of cake.
Parallel keys are keys with the same tonic.
For example, C major and c minor are parallel keys. When arranged consecutively in alphabetical order, the pattern of half and whole steps typically identifies a particular key tonal system based on the major and minor scales. Scales tend to gravitate toward a single pitch, indicating the tonic pitch. A key signature is a means of providing the correct accidentals for the scale being used, positioning the half and whole steps in the appropriate arrangement.