What is a Relative Minor ?

Any Major chord has a Relative Minor chord - conversely any Minor chord has a relative Major.
To find the Relative Minor of any major chord you simply play the chord as a barre then drop down the neck four frets counting the one you're on.
You then 'Minor' the chord - i.e. play the minor version of that chord.
I have put a soundfile of this starting on a C Major at the 8th fret ending up on A Minor at the 5th fret. See the tab also.

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The Tab shows the moves to find the relative minor of a Cmajor chord

A Minor is the relative minor of C Major.
D Minor is the relative minor of F Major.
E Minor is the relative minor of G Major.

The three chords pictured below are the relative minors of the three chord trick in the key of C Major.

They all belong in the key structure of C Major.

Note that the root notes of the three minors follow the same pattern as the three majors - i.e. straight across then up 2 frets (follow the red dots).

We now have SIX chords in the Key Structure of C MAJOR - three majors and three minors.

Exercise
Work out the six chords for as many other keys as you can. You will find that to drop down four frets from , say , F# at the 2nd fret is impossible.
If your guitar can be played at fret 14 work it out from there - if not find F# rooted on the 5th string at fret 9 and work it out from there.