How is B#°7 the vii7 of E?

[ClairDeLune]

Chopin's Op. 64 No. 2, the Waltz in C# minor, has an interesting analysis I found on YouTube. In measure 6, it states that it's in A major with a pedal point on E, starting with the chord E, F#, A, D#, and B#. I'm a bit puzzled because shouldn't the vii of E be a D# chord instead of B#? Any insights would be appreciated!

[BluesGuy]

When you think about it, diminished chords like B#°7 have this unique tension that makes them feel a bit unstable, which is perfect for leading into something else. In this instance, the pedal point on E adds that bluesy tension, enhancing the emotional pull of the piece. You’re right to feel confused, but the way these chords work together can create a beautiful harmonic journey.

[GearNerd]

It's important to note that B#°7 and D#°7 are enharmonically equivalent, meaning they are essentially the same chord but spelled differently. The notes for B#°7 are B#, D#, F#, and A, which can also be interpreted as D#, F#, A, C (B#). This symmetry in diminished chords often leads to different spellings for clarity in context. In this case, it's possible that the analysis uses B# to maintain the harmonic function in this passage.