Why On Earth Would You Ever Modulate From C To B#?

So I bought Max Reger's Modulation, a book containing simple examples of modulation in four-part harmony. I'm skimming through it and it definitely seems like it will be helpful, but I did a double take seeing an example of modulation from C major to B# major.

I just don't really get what it would be used for.

Edit: corrected from "Max Weber"! Sorry about that.

I searched the book you're talking about (although it was a bit difficult, as I was first looking for a "Max Weber's" work, and I the results we all about sociology....). Well, in Max REGER's Modulation, I believe the C-major-to-B#-major modulation may work as a turn-around, more like static harmony, like a chord progression that comes back to C major.

But, if you're working with non-tempered systems, it could end up as a valuable tool, I guess ^^

Very simple, say you want to move from B flat major to C# major. You modulate to C major and then through an enharmonic modulation you move to B # major which serves as a VII of C# major.  Or say you are in C major and and managed to get your self to G # minor. You could easily get to a B sharp major chord that could be enharmonically respelled later as C major as you try to return to this key.

It is esoteric and useless if you take Reger's modulations at face value - that is employed for 2 - 4 bars literally (and I do believe there is a bit of tongue in cheek in that book). These are rather models which Reger offers the composer to employ imaginatively in his own compositions.  For the example I gave,

More sudden enharmonic modulations occur in a few pieces of Chopin - a famous example is one of his waltzes where within just a few measures moves to the #V of A flat major and stays there for a little bit before respelling the chord as the flat VI. .  

The only plausible reason would be to make use of an enharmonic spelling, introducing a pivot chord for modulation. However, since ideas on how modulation should be achieved are different from what they used to be (key changes which seemed unnatural to listeners in the 18th century seem natural now) the situation is very unlikely to arise. Personally I would be unhappy to notate in B-sharp major, especially when considering the fact that the relative minor would be G-double-sharp minor - I'd want to find a different notational solution!

CO already gave examples, but I think it boils down to where you need to go. I doubt it'd be of much use to finish a modulation in just those two steps, but if B# is supposed to be considered within a different harmonic context that allows for further modulation (such as a mediant or whatever related to a destination that is related to B# harmonically) then just writing C "twice" would be, well, lazy. Easier to read maybe, but lazy since it's not what you're really doing in that case.

The other point of course was already made about enharmonics. I'm sure Liszt used stuff like Cb major as bridge more than once for the same reason, it'd be easy to figure out scenarios where you could do the same with c->b#.

Sounds like a preliminary step to making a further modulation that would be awkward from C, like G# or D# or something. I don't know. It's late.

Yeah, pretty unlikely though. I do recall there were several late romantic composers that refused to respell enharmonically and used triple and quadruple flats extensively in pieces that moved around harmonically a lot. So, I'm sure there are examples out there.

Wouldn't worry about it to much, if you need to use the technique I'm sure it will make sense to you within the context when it comes up.

I mean, once you've seen a Cbb-minor key signature you kinda just stop questioning these sorts of things.