They don't, except for when they do. So I want to clarify what I said about Bach not writing the Toccata and Fugue in d minor. There's been a lot of discussion on the subject, and from everything I've gathered the answer is that this piece is one of the more anomalous in Bach's repertoire which, along with the lack of an original score written in Bach's hand, makes his authorship of the piece seem somewhat less that certain by a handful of Bach scholars. However, until there is more substantive evidence, the convention is to attribute this piece to Bach. So "Bach didn't write the toccata and fugue in d minor" may have been a little too strong here, maybe even misleading, and I just wanted to take a minute to completely clarify that point. The quote from Jean-Claude Zehnder is "The matter still remains open, despite the scholarly discourse that began in 1981. Until proof of the contrary, BWV 565 should be considered as a work by Johann Sebastian Bach." oops.