How does bV7b5 chord fits in both major key and minor key?

Question

In key of E major going from B7sus2/4-->Bb7b5-->Amaj7 works.

In key of C#minor going from G#min7-->G7b5-->F#min7 works.

Can someone explain why?

Answer 1

The tritone substitution can be performed by exchanging a dominant seven chord for another dominant seven chord which is a tritone away from it. For example, in the key of C major one can use D♭7 instead of G7. (D♭ is a tritone away from G).

https://en.wikipedia.org/wiki/Tritone_substitution

The progression you’re mentioning is the usual passing chord in the „12-bar blues pattern“ in measure 9:

V - IV (G - F): (the passing chord Gb is bV7 = tritone substitution of the secondary dominant C7 =>

I7 - IV (C7 - F) = V7/IV whereby V7/ (slash!) means: secondary dominant

Tritone: Gb is a tritone (diminished 5th) away from C

as I7b5 = bV7b5

Answer 2

The core of why these work is the descending chromatic root movement, which is helped along by the chromatically descending sevenths. That is to say, the outer voices, which dominate aurally (in root, close position), move in parallel by descending half-steps.

To illustrate, I'll transpose both progressions to have the same roots.

X:0
K:C
L:1/2
[CDFG_B] [B,^D=FA] | [_B,DFA]2 || [C_EG_B] [B,^D=FA] | [_B,_DF_A] ||
s: C7sus2/4 B7b5 | Bbmaj7 | Cmin7 B7b5 | Bbmin7 ||

But, now suppose we reinterpret the B7b5; we get,

X:0
K:C
L:1/2
[CDFG_B] [_C_EFA] | [_B,DFA]2 || [C_EG_B] [_C_EFA] | [_B,_DF_A] ||
s: C7sus2/4 F7b5 | Bbmaj7 | Cmin7 F7b5 | Bbmin7 ||

It's now clear that the second chord in each progression is just a dominant chord in disguise.

But to visually reinforce the "how/why it works", here is the chord progression stripped down to the parallel sevenths.

X:0
K:C
L:1/2
[C_B] [B,A] | [_B,A]2 || [C_B] [B,A] | [_B,_A] ||

Answer 3

In key of E major going from B7sus2/4-->Bb7b5-->Amaj7 works.

In key of C#minor going from G#min7-->G7b5-->F#min7 works.

Both have root descending by half steps and then altered dominant seventh chord moving to what are plausible tonic chords or tonicized chords.

The Bb7b5 and G7b5 sound like tritone substitutions.

It's hard to say much more. Nothing really indicates the keys you've given, but assuming something else establishes those keys, you could say Amaj7 and F#min7 and temporarily tonicized and these are tritone substitution progressions.

The generic pattern is ii bII I or a bit more specifically iim7 bIIalt7 I in major or iim7b5 bII7 i in minor. There is a fair amount of flexibility with the exact chord qualities. The ii will get a minor third and minor seventh, the bII will get a major third and minor seventh. Both chord's fifths are then the flexible degree that hints at major/minor mode. The i/I chord can take various extensions or additions. So it's roots on descending half steps to the tonic with the penultimate chord being some kind of dominant seventh chord.