In the key of Em, does the F#m7-5 chord have the b13 (D natural) or the 13 as an available (ex)tension?
Same question in Bb Major regarding the Gm7 chord, is the 13 (E natural) an available (ex)tension?
If so, why??
In the key of Em, does the F#m7-5 chord have the b13 (D natural) or the 13 as an available (ex)tension?
Same question in Bb Major regarding the Gm7 chord, is the 13 (E natural) an available (ex)tension?
If so, why??
For half-diminished 7th chords, the b13 is an available tension and the natural 13 and the #13 are not available (technically the #13 is just an octave above the 7th so it's not a tension in this case). So for the F#m7-5 chord, D natural is available and D# is not. The reason why the b13 is available in this case is because the 5 is diminished, which means the b13 is two steps away from both the 5th and the 7th.
For minor 7th chords, the natural 13 is an available tension, the #13 is enharmonic to the 7th (an octave up) and the b13 is unavailable. So you could use E natural for the Gm7 chord. The reason why the b13 is unavailable in this case is because it is only one step from the 5th. The 13 is two steps from the 5th and only one step from the 7th, but it works better to be closer to the 7th than the 5th in this case.
See: http://www.thejazzpianosite.com/jazz-piano-lessons/jazz-chords/available-tensions/