Like the children who mask their identities while trick or treating, Halloween's most famous music is in disguise.

		Bach's Toccata and Fugue in D minor Listen to MP3 files of excerpts from Bach's Toccata and Fugue in D minor from "Bach: Great Organ Works," Helmut Walcha, organ; Deutsche Grammophon. Opening sequence Initial statement of the fugue subject

For at least a century, Johann Sebastian Bach's memorable organ work, the Toccata and Fugue in D minor, has embodied the eerie and the supernatural.

You've all heard the spiky opening half trill and ensuing demonic descent. It's enough to send a chill down the spine: De-de-Deeeee, De-de-de-de-deee-De.

Films such as "Dr. Jekyll and Mr. Hyde" and "Tales From the Crypt," not to mention countless cartoons, use it to set a spooky ambiance. It has even invaded the musical realm, adapted by Andrew Lloyd Webber as the main theme of "The Phantom of the Opera." Many also will remember its appearance in Disney's blockbuster film, "Fantasia," and you can hear it at Mellon Arena when the Penguins' organist reflects a malevolent turn of events on ice.

Turns out Halloween's soundtrack also has been cloaking its true form: The Toccata and Fugue probably was not written by Bach and almost certainly wasn't written for the organ.

In music circles, that assertion is as scary as it gets.

It's not every day such a famous work gets shaken to its foundations. However, scholarly consensus is building that the baroque master did not write his most well-known organ work.

Solving the mystery

The clues lie in the music. For one, Bach's manuscript copy of the Toccata -- the handwritten original -- is lost, if it ever existed. That means attribution can't be certain; it's akin to trying a murder case without a dead body.

Like a good mystery, the sources are questionable, too. The earliest copy of the Toccata was done by a man with a reputation of passing off spurious works under Bach's name.

However, the biggest questions arise when the Toccata is examined stylistically.

"It is a little worrying when literally the first and last notes of a piece of music raise doubts," writes Peter Williams in a seminal article about the Toccata in the journal Early Music in 1981.

Williams' argument stems from stacking up many small odd points about the piece. There's the name -- Bach's generation would have called it "Praeludium et fuga," not Toccata and Fugue -- and a progression of notes Bach never would have allowed.

"Bach's greatest inspiration is invariably revealed through his complete mastery of the 'rules,' " writes Bruce Fox-LeFriche, who raised the Toccata subject again last year in Strings magazine.

The evidence of rule-breaking includes doubling at the octave and the curious minor cadence that ends the Toccata, both not heard elsewhere in Bach's organ output (usually even a work in a minor key concludes with a major chord). The Toccata also brims with harmony and counterpoint bordering on simplistic for the masterful composer.

"No other Bach fugue contains such feeble part-writing," writes Fox-LeFriche, citing the "complete absence of contrasting rhythm, contrary motion or a least a few notes that don't slavishly follow the subject."

In short, the Toccata and Fugue approaches nothing Bach ever wrote for the organ, or ever wrote at all.

"It is certainly very different than any of his organ works," says Don Fellows, organist at St. Paul Cathedral in Oakland. "There are parts that don't fit the hands."

Some argue that the Toccata was the product of a young and adventurous Bach. Chief among them is venerable scholar Christoph Wolff, whose painstaking attempts in his "Bach: The Learned Musician" to explain away its oddities and keep the work in Bach's canon only reveal what a stretch it actually is. It's not that Bach wasn't capable of writing music out of the norm (he did plenty of that), it's that this Toccata flies in the face of his basic principles.

Compared to learned Bach in compositions such as "The Art of Fugue" or "The Well-Tempered Clavier," the Toccata and Fugue in D minor is facile, dramatic and eccentric.

And, if past evidence can be used against him, this would not be the first Bach work that is now considered to be by someone else: The keyboard Minuet in G and the aria "Bist du bei mir" are some of the others.

Removing the mask

Scholars now think the Toccata was originally a violin piece Bach transcribed.

"If you know the piece you can just see it was written for the violin," says Don Franklin, a Pitt musicologist specializing in the composer. "It has idiomatic figuration for the violin [and] the initial statement of the fugue subject can easily be played on the D string, crossing over to touch the G string."

The opening of the Toccata, too, is violin-like, offering "the solo violin an opportunity to drop down through its four strings," writes Williams. And there are other nuances that add up to an organ piece covering up its origins.

This hypothesis fits. "Bach did a lot of transcription," says Franklin, also past president of the American Bach Society. Perhaps this Toccata simply lends itself to transcription. After all, Leopold Stokowski's orchestral version worked out pretty well in "Fantasia" and in concerts.

The evidence all points to the fact that Toccata does not match organ music of the time, especially Bach's. It does fit the period's string music, however.

"I don't know what it was written for, but many suspect that the violin is the next obvious instrument," says famed baroque violinist Andrew Manze. For those in the know, just think of the rapid alteration and quick runs of Vivaldi and Corelli string concertos, and you get the picture.

Manze, Williams and Fox-LeFriche are among those who have attempted to turn this werewolf of a work back into its original form, although transposed to A minor. "I tried to constrict myself to the sort of violin writing that Bach used himself," says Manze, whose version is found on the Harmonia Mundi recording "Portrait."

Most telling? While many consider the Toccata a somewhat tricky organ show piece, Manze says just the opposite about it on the violin: "It is not a terribly difficult piece to play."

Does it really matter?

Some skeletons in the closet can return to destroy the present -- such as the telltale beating heart of a victim buried under the floorboards or that long-dead mom hidden upstairs in a psycho's house. But the Toccata lives on as an organ piece. No matter how it got to the keyboard, it turned out to be a good thing. Its transformation is a happy laboratory accident that created a Mr. Hyde we all want to have around.

"You can still play it on the organ and appreciate it," says Franklin.

Music often gains other meanings. Certainly neither Bach nor the original composer intended for the work to have the ambiance of Gothic horror it now has. For that matter, they would not have dreamed the venerable church organ could ever gain a demonic association as a harbinger of evil.

And this Halloween, would you rather hear the Toccata on the mighty organ or the charming violin?

In the end, it matters less who wrote the Toccata and Fugue or what instrument was intended, but more that it still thrills us today. We just might want to remember that, like much on Halloween, the Toccata and Fugue in D minor is not entirely what it seems.