Minor chords are constructed when the major chord formula is altered from R, M3rd, P5, to R, ♭3rd, P5; this flat third is the key distinguishing feature of a minor chord. It should be duly noted, however, that a minor chord is still built from the Circle of Fifths, mimicked by the respective Major Scale minor chords are not built from Minor Scales.
Although an E♭ is found in a C Minor Scale, one should be careful to take notice that the minor chord formula dictates that you flat the third. Since the notes of a C Minor Scale are C, D, E♭, F, G♭, A♭ and B♭. If you flat the third of a C Minor Scale, you would produce a double E-flat (E♭♭) and not the prescribed E♭ found in a Cm chord (C, E♭, G).
Minor 6th are chords consist of the 1st-♭3rd-5th-6th intervals, while minor 6/9 chords are chords consisting of the 1st-♭3rd-(5th)-6th-9th intervals.
Minor seventh or m7 chords are formed by adding a flat seventh to a traditional minor chord, R, ♭3, P5, ♭7. So a Cm7 is C-E♭-G-B♭, as with the traditional minor, the third remains flat. If you were to return the third to a major third, you would effect a dominant seventh chord and not a minor seventh.
Minor ninth chords or m9 are built much like a minor seventh, but ninth interval remains natural (R, ♭3, P5, ♭7, 9) only the third and seventh interval are flat, however, the first, fifth and ninth intervals remain unchanged. Additionally, minor eleventh chords (m11) are constructed as the minor ninth, and include the eleventh extension (R, ♭3, P5, ♭7, 9, 11).
Since there are six intervals in these chords, one may omit the ninth for accommodation’s sake (on some instruments, it may be too difficult or impossible to simultaneously play all intervals).
Minor thirteenth or m13 contain the root, flat third, fifth, flat seventh, ninth, eleventh, and thirteenth (R, ♭3, P5, ♭7, 9, 11, 13). However, as with the minor eleventh, the ninth can be omitted, as well as the eleventh, in order to accommodate certain instruments remain unchanged. Additionally, minor eleventh chords (m11) are constructed as the minor ninth, and include the eleventh extension (R, ♭3, P5, ♭7, 9, 11).
Minor/major seventh or min/maj7 chords contain a flat third, a perfect fifth, and a natural seventh (R, ♭3, P5, 7). However, they are usually considered to be minor chords even though their chord structure shares characteristics from both the minor and major families (i.e. a flat third found in the minor chord family, and a natural seventh found in the major chord family).