I recently read that the 5-note 7th degree chord of the C-minor melodic scale is Bm7b5b9. By the 'rule of thirds' the notes in this chord by my deduction would be B-D-F-A-C, but this only works if one uses the B-minor natural scale. So, is my deduction correct? if so, is it a given that one always uses the natural scale when deduceing the notes of a minor chord?
Yes. A nice thing about chord symbols is that their realization as notes is the same regardless of keys or scales. Otherwise it would be quite awkward to write and read a "lyrics and chord symbols" accompaniment sheet. :) But now because the chords are absolute, you can accompany a song without knowing about scales and keys.
Edit. As Michael Curtis says (in other words), you don't need to think about different scales depending on whether it's a minor or major chord. The minor/major aspect only decides the 3rd of the chord, and the rest of the numbers/tensions work the same, regardless of what the third is.
You can also think of any chord symbol as relative to a dominant chord. Symbols for quality like
Δ, etc. and accidentals on numeric figures like
b9, etc. are all relative to a dominant chord in a major key with potential extensions up to
Bm7b5b9 is like
B9 but the base triad is minor, the fifth is lowered, and the ninth is lowered. The seventh doesn't have a modifying accidental so it is the same as the dominant ninth: a minor seventh.
Instead of trying to match a chord quality to a scale type, thinking of chord symbols relative to a dominant chord treats all the chord symbol figures as intervals above the root. So a flat nine really means 'half step lower' relative to the dominant ninth in major which is the interval major ninth. Flat nine means minor ninth - in this case a minor ninth above a root
In the dominant chord the intervals are: M3 P5 m7 M9 P11 M13, where M=major, m=minor, and P=perfect.
I don't know if this will seem convoluted to you, but it's a way to figure out the specific tones of a chord relative to the root using only one point of reference, the dominant chord.
Another way to go about realising this chord symbol:
- Start with the root. It's B.
- What kind of seventh chord is it? Here, it's Bm7: B-D-F♯-A.
- What do those alterations and extensions do? The ♭5 means we take that F♯ down to F♮, and the ♭9 tells us that 1) we have the ninth (some kind of C note) and 2) we happen to have a specific kind called the flattened 9th (For the root B, it's C♮).
- Put 'em together and what have you got? No, not "Bibbity Bobbity Boo" - You're now the proud owner of the notes B-D-F-A-C, spelling out your Bm7♭5♭9 chord!
As others have pointed out, any extensions implied by the chord symbol but not explicitly altered should be diatonic to the dominant scale built on the root, regardless of the prevailing key of the music. As an example, E9♭13 contains the notes E, G♯, B, D, F♯, and C, regardless of whether the chord is played in C major or E major (or in any other key center).
You can think of a chord symbol as a way to describe how far the chord tones should be placed from the chord root - e.g., an "m" in a chord symbol tells you the third is closer to the root than it is in a symbol that lacks an "m".
Since it depends on measuring how far the tones are from the root, we need a consistent yardstick. The problem with minor scales is they aren't consistent: the distance from the tonic to the sixth is different in a natural minor scale than it is in a melodic minor or Dorian scale, and the seventh is different if you're using the melodic or harmonic instead of the natural minor.
Since there's only one kind of major scale, that's our consistent yardstick. We match the tonic of the major scale to the chord root, and then the chord symbol parts tell us exactly which tones to use ("m" to lower the 3rd, "+" to raise the 5th, etc.)