Dominant 7th flat 5 - Dominant 7th flat 5

Here the compared chords respectively are identical. Each one is built on a root which is the b5 of the other. (rd)

A7b5 = Eb7b5
B7b5 = F7b5
C7b5 = Gb7b5
D7b5 = Ab7b5
E7b5 = Bb7b5
F7b5 = Cb7b5 (B7b5)
G7b5 = Db7b5

Because of the above relationship of the 7b5 chords, it is possible to substitute A7 for Eb7, B7 for F7, etc in certain situations. I call these pairs alternate 7ths of each other as they might be substituted for each other. (ie:) in Dm7 to G7 to Cmaj7 use Dm7 to Db7 to Cmaj7 providing it satisfies the ear. (rd)