If we take the scale of C Major from C to C':
A Bb B | C Db D Eb E F F# G Ab A Bb B C' | Db D' Eb E' F' F# G' Ab A'
It ascends from C to octave C (C') by the intervals of the Major Scale (TTSTTTS*). But were we position the minor scale intervals against the C major scale, and adjust them to fit without sharps or flats, you would find that they did so at one position - starting on the note of A:
A Bb B | C Db D Eb E F F# G Ab A' Bb B' C' | Db D' Eb E' F' F# G' Ab A'
So there is a Minor Key with no sharps or flats, and it is A minor ( or Am ). Because it has no sharps or flats, it is similar to the Major Scale of C and so it is known as the Relative Minor< of C. Note that the starting note of a Relative Minor is always 3 semitones below the Major Tonic (starting note), so we have:
In chord progressions, going from the Root to the Relative Minor is a pleasing effect and used in many tunes. e.g. From the G Major chord to the E Minor chord.