At least one of us is very confused. I don't think it's me.
At the end of the toccata there is a chord containing the following notes, from bottom to top: D (in the bass, on the pedals), another D (lowest note on the manuals), F, A, D. This is a perfectly ordinary chord of D minor, of course. After that there is a semiquaver rest and then the fugue subject begins (or, perhaps better, the fugue subject begins with a semiquaver rest). At that point, as is normal in a fugue, there is only one voice sounding.
Oh, wait, you weren't talking about the fugue at all? You meant the chord a few bars into the toccata? Well, OK then, that chord contains the notes you said it does. (Though, I repeat, it isn't "the chord held at the start of Bach's Fugue in D minor"; it's in the toccata, not the fugue; in a discussion of music analysis such distinctions are really worth making.)
But there's nothing weird about that chord! It's a standard diminished-7th chord (everything at intervals of 3 semitones from some starting point; in this instance C#, E, G, Bb). If I may quote from that bastion of the avant garde, Wikipedia:
The most common form of the diminished seventh chord is that rooted on the leading tone [...] These notes occur naturally in the harmonic minor scale.
Diminished seventh, check. Rooted on the leading tone, check. Minor key, check. It's perfectly commonplace. (There are plenty of much weirder things in Bach.)
The chord is in measure 2 of the piece, and contains these notes: D, C#, E, G, Bb, C#, E.
A diminished 7th in Dm should have D, F, Ab, Bb, shouldn't it? This is a diminished 7th C#, so what's the D doing there?
Anyway, my impression is that diminished 7ths are much more common in organ music than in piano music. I think of them as "that organ-music chord". And if you look up diminished 7th in the same music database that komponisto linked to above, you'll see it has a much higher fraction of baroque entries than any of the other items on that list....
We recently established a successful Useful Concepts Repository. It got me thinking about all the useless or actively harmful concepts I had carried around for in some cases most of my life before seeing them for what they were. Then it occurred to me that I probably still have some poisonous concepts lurking in my mind, and I thought creating this thread might be one way to discover what they are.
I'll start us off with one simple example: The Bohr model of the atom as it is taught in school is a dangerous thing to keep in your head for too long. I graduated from high school believing that it was basically a correct physical representation of atoms. (And I went to a *good* high school.) Some may say that the Bohr model serves a useful role as a lie-to-children to bridge understanding to the true physics, but if so, why do so many adults still think atoms look like concentric circular orbits of electrons around a nucleus?
There's one hallmark of truly bad concepts: they actively work against correct induction. Thinking in terms of the Bohr model actively prevents you from understanding molecular bonding and, really, everything about how an atom can serve as a functional piece of a real thing like a protein or a diamond.
Bad concepts don't have to be scientific. Religion is held to be a pretty harmful concept around here. There are certain political theories which might qualify, except I expect that one man's harmful political concept is another man's core value system, so as usual we should probably stay away from politics. But I welcome input as fuzzy as common folk advice you receive that turned out to be really costly.