At least 10 percent of the harpsichord makers in the United States -- two of perhaps 20 -- live and work in an old firehouse several blocks south of Dupont Circle.

Tom ics and part musicians -- not to mention detectives and historians. They have spent years studying and restoring original harpsichords, clavichords and fortepianos in order to understand how to make them themselves. They know their instruments so thoroughly that they can often identify an unsigned one just by the maker's tool marks.

"It seems totally bewildering to someone just casually looking into it," says Tom. But well-trained instrument makers "can really recognize the individual quirks of how a man finished off with his knife or finished off with his gouge. Everybody's got his own individual way of sharpening the chisel and his own funny way of being able to handle it to make it work," he says.

Their first-floor workshop, formerly the garage for the fire trucks, is cluttered with half-finished instruments, pieces of wood, electric saws and other tools that bring yet another occupation to mind.

"Every now and again, I feel the doctor analogy coming up," says Barbara. "When you're working on an instrument, whether it's new or old, you're in there with little surgical tools and knives and you're sort of probing, poking and futzing in order to make it better. And you do literally X-ray things if you can't get in to see it."

Neither of the Wolfs planned to make instruments. After meeting in high school at a music camp in the Midwest, both ended up at the New England Conservatory several years later -- Tom studying double bass, Barbara, piano. After the first year of study, Tom heard about some summer jobs in several Boston instrument shops and decided to apply.

"I simply got on the subway -- the first shop was in Cambridge, the second was in Waltham. I got as far as Cambridge and got hired," he says. He also got hooked and encouraged Barbara to try it, and they haven't done anything else since. Both apprenticed in several shops for five years and moved to Washington 12 years ago after Tom got a job with the Smithsonian. Shortly after, they set up their own shop.

At the back of the shop is a small office with a cabinet holding skeins of iron, steel and brass wire that will someday be instrument strings. A glass door separates the office from the small kitchen -- an attempt to separate private life from professional, though it doesn't always work.

"In the beginning we were wandering in and out of the office constantly, at all hours. We've gotten better," says Barbara.

"Most people who work outside their homes aren't really that interested in hearing what went on at their spouse's office," says Tom. "But if one of us has an appointment outside, we really are interested because it is so different."

Maybe that's why they particularly enjoy meeting and working with their diverse customers. Some, like Christopher Hogwood, are professional musicians; others institutions like the Kennedy Center; and still others run the gamut from astronomers to, as Barbara puts it, "divorce's who say, 'I could probably spend this money on a therapist but I think I need a harpsichord more.' "

Although both are competent in all aspects of early keyboard instrument building, Tom tends to do most of the woodworking and designing while Barbara does most of the musical finishing -- the delicate adjustment of the parts once the instrument is assembled, which gives it its voice -- and the decorating.

"We can both do any of it if we have to. But in the long run you come down to being efficient. Whoever is going to be able to get it out at the moment is really who does it," says Barbara.

But building a top-quality instrument is their greatest concern.

" The instrument really represents you for the next 200 or 300 years. You have to be able to stand behind what you do. You can't cut corners," says Tom.

Not cutting corners means spending about 800 hours building a harpsichord, which translates into a 1 1/2-year wait for customers. The instrument must first be drawn on paper, much the way an architect draws blueprints for a building. Then the hundreds of components have to be cut out, varnished and painted. The case and soundboard are the easiest; jacks, hammer shanks and other action parts on harpsichords and fortepianos take much more time.

"We always try to do some restoration work every year just to keep our perspective on things," says Barbara. "You can really see the trouble to which that person did or didn't go."

Even when they've taken the greatest care, they don't know how an instrument will sound until it's completed.

"We always get kind of hyper and frantic at this point the musical finishing because you really just want to sit down and see what it's going to do for you," says Barbara. "I guess that's one of the things that makes them feel alive for you. It's like finding out how they talk."