How one of the most esoteric musical works ever written became an unlikely hit.

When I tell musical friends that on its release in March, Pierre-Laurent Aimard's solo-piano version of J.S. Bach's The Art of Fugue shot to the top of the Billboard and iTunes classical charts, they get this glazed look. It's as if you told a physicist that Einstein's General Theory of Relativity was topping the best-seller list. It's not supposed to happen.

This is because the 14 fugues and four canons that make up The Art of Fugue constitute one of the most esoteric musical works ever written. Each fugue bears the severe title Contrapunctus followed by a number, and there is no indication of what instruments are supposed to play them. Every piece is in D minor; all are based on the same melodic theme. It's as if Bach intended the AOF as a theoretical treatise, to be read and studied rather than performed, to demonstrate some of the more arcane things you can do with the idea of a fugue.

Surely nobody expected this recording to take off. French pianist Aimard made his name playing 20^th-century repertoire on the order of Ligeti, Messiaen, and Ives. This was his first release for the prestigious German label DGG, which must have wondered about his marbles when he declared The Art of Fugue to be his choice. The label made no particular PR push on behalf of the recording, and, anyway, selling classical music is a patchwork business. I've seen the classics sold as rock 'n' roll ( Beethoven the Revolutionary, the liner notes putting him up there with Elvis and Kurt Cobain); sold like barbecue ( Grillin' And Chillin' with Johann Sebastian); sold like Cialis ( A Ravel Weekend); and, in the case of violinist Anne-Sophie Mutter and her fetching album covers, sold in the fashion of Pamela Anderson and other famous mammals.

A while back there was a brisk trade in Bach synthesized, scat-sung, plucked on koto and guitar, etc. Many of those ended up in my record store's remainder bin labeled "Schlock Bach." In contrast, Aimard's piano rendition of The Art of Fugue is straightforward and sober, trying not to let on how formidable the fugues are to play with 10 fingers. Organ, with its foot pedals, is the more logical keyboard choice; most often the AOF is done with multiple instruments. The dozens of recordings listed on Amazon include versions by the Canadian Brass and the Berlin Saxophone Quartet.

Before I give a tour of the piece, there's an issue to deal with: One has to assume that most of the people snapping up Aimard's The Art of Fugue don't actually, as it were, know what a fugue is. Here's the short course: A fugue is a contrapuntal procedure. ... Wait, you may not know what counterpoint is either.

Counterpoint is an ancient way of writing music in which everybody is singing or playing melody at the same time, rather than the relatively more modern and familiar idea of a single tune with accompaniment: guy with guitar, soprano with piano. Counterpoint is the art of juxtaposing melodies so that instead of getting in one another's way, they complement one another and make good harmony together. As any music student will tell you, counterpoint is damned hard to write, and the requirements of fugue only make it harder.

A fugue generally begins with a bit of tune called the subject, played alone in one voice (in counterpoint every part is called a "voice," whether it's sung or played). Then another "voice" strikes up the fugue subject while the original voice continues in counterpoint, sometimes establishing an also-recurring tune called the countersubject. The fugue carries on, in two to five or more voices, with entries of the subject plus new melodies woven freely around it. Sections featuring the subject alternate with episodes of free counterpoint where the subject gets a rest. So in a fugue, the subject is like a character who keeps turning up in a conversation, perhaps with spouse along (the countersubject). Except that in a contrapuntal conversation, everybody is talking at once, yet, magically, it all makes sense.

For an example, here's the beginning of Aimard's Contrapunctus 1. As the subject you'll hear the famous Art of Fugue theme, then three more entries of the subject until we're in four-voice counterpoint.

Fugues have been around for centuries, and lots of composers have written them, but few have created fugues as complex, disciplined, and beautiful as Bach's. It's the beauty that boggles musicians: There's a kind of mathematical elegance about them, but math doesn't sound that good. As a performer, Bach could improvise multivoice fugues at the organ, which is like writing four to six prose essays at the same time, using both hands and feet. It's not that he couldn't write a nice, straightforward tune. Just about everything that's possible to do in music, Bach could do as well as, or better than, anybody else. But he loved leaping self-imposed technical hurdles, the more fiendish the better. Thus The Art of Fugue.