I have had some fun with guitar buddies in the last few years finding ways to use this flexible chord and it's analysis is worthy of some thought (in my opinion).
Firstly, if you are talking about major key harmony, it is your diatonic sub for your V chord if you build it off the seventh degree of your scale. ie: In the key of C Major, the Bm75 is the diatonic sub for your G7, or your V chord. The notes of your G7 chord are G B D F; the notes of the Bm7b5 are B D F A, giving us a G9 (less the root), also called a 9 for 1 sub.
Secondly, if you are talking about minor key harmony, the m7b5 is the natural ii chord. ie: In the Key of Am, Bm7b5 is your ii chord. Your ii V i progression being Bm7b5, E7(b9), Am7. This is perhaps simplifying things a bit, since a minor key may contain different chords depending on the scale you have built your chords around, but simply put, a m7b5 built a tone above of the minor chord you are approaching will precede the V7 of the minor chord you are approaching for a consonant resolution.
(http://www.musilosophy.com/chord-charts.htm has various minor key harmony charts. Note in a melodic minor you have a minor ii chord and a m7b5 as a vii chord.)
Thirdly, if you compare the notes of this chord to the other chord notes we find that if D is our root we have D F A B or Dmin6. (One man's m7b5 is another man's min6.)
Likewise, if we consider A our root, we have A B D F, (R, 9, 11, #5) - lots of colour.
If we consider the F the root, you have an F A B D, or F with a major 3rd, b5 and a 6th...(whatever you want to call that one).
This is one very colourful chord and has perplexed me for years.
This is just the beginning. Take a look at m7b5 subs for blues progressions:
If you build a m7b5 off the 3rd of your dominant 7 chord, you get great color... a Bm7b5 (B D F A) for a G7 (G B D F) - (same as above for the G9 sound); likewise, a Em7b5 (E G Bb D) for a C7 (C E G Bb) or an F#m7b5 for a D7. I find the m7b5 built on the 3rd of the IV chord (or a tone and a half back from the key you are playing) is very flexible, or the Em7b5 in G Blues. Note that your blues scale in G would be G, Bb, C, Db, D, F, so we are stretching a bit, but that is half the fun; isn't it?
Anyone have any more ideas about this chord and its possible substitutions?
These are moveable m7b5 chords, so you will have to be able to find your root notes.
Roots on EADG string set
Cheers,
SarinaJ
http://www.guitartricks.ca
