The formula for the construction of a minor chord is 1 b3 5 i.e., a minor chord consists of the first, flattened third and fifth notes of the major scale. It must be remembered that this formula, although used to construct a minor chord, is still based upon the notes of the major scale.
Example 1: The C minor chord (Cm) consists of the first, flattened third and fifth notes of the C major scale (i.e., C, Eb, G).
C Major Scale
Example 2: The Am chord consists of the first, flattened third and fifth notes of the A major scale (i.e., A C E).
G Major Scale
Thus the only difference between the major chord and the minor chord is the flattened third note.