Before I visited Ronald Jonas Interiors on West 18th Street, I was under the impression that upholsterers simply made seat cushions, and possibly the occasional pillow. An afternoon with Steven Jonas and his brother-in-law Charles Berlin, the owners of this third-generation family-owned firm specializing in upholstery and curtain-making, quickly disabused me of any such notions. High-end upholsterers such as Jonas Interiors are jacks-of-all-trades, creating not only cushions but everything those cushions rest on – from a couch’s solid wood frame to the detailing on a chair leg. And Jonas Interiors has earned a reputation as one of the finest custom-made furniture makers around; through the years, it has restored furniture at Versailles, created a chair for president Kennedy specifically designed for his bad back, and built pieces for Carl Icahn, Steven Spielberg, and Ralph Lauren.

“We rip our own lumber,” Mr. Jonas told me as we passed through the sunlit showroom, crowded with floor samples of sofas and club chairs covered in white muslin. “We build it all up from scratch. This is the way it’s always been done.”

At least in the Jonas family. The firm has its roots in Salzburg, Austria, where it began in about 1912, started by Mr. Jonas’s great-uncle. “He was one of three brothers trained as architects. In those days, they all did their own in-house work,” Mr. Jonas said, explaining that they built furniture specifically de signed to fit the space. Josef Hoffmann, the noted Austrian architect and designer, was also a partner. Several family members, including Mr. Jonas’s father, Ronald, settled in New York after World War II, and reopened the business uptown in German speaking Yorkville; over two decades ago they moved to their current location. “The business has been in the family forever and it will continue to be,” Mr. Jonas said. “I want my daughter to take it over.” (She’s currently 16.)

In a back room, filled with wooden frames piled high atop one another, Mr. Jonas pointed to a maplewood skeleton of a chair. “See, there are no knots; all of our wood is kiln-dried,” he said with pride, explaining that each chair joint is secured with dowels in fitted corner blocks and screwed into the frame. “All of this will be covered up, but that doesn’t matter. Even if you don’t see it, the wood is there. It’s solid.”

Mr. Jonas, 47, learned such attention to detail from his father. “I started from the bottom up. My father had a real old-world European mentality. When I was 16, I said I didn’t want to go to camp anymore, and he said, ‘Fine. You’re going to come to work with me.'”

Years later, Mr. Jonas pulled his brother-in-law, Charles Berlin, into the fold. Mr. Berlin, who is married to Mr. Jonas’s sister Sharon, trained as a corporate lawyer, and worked for five years in the insurance department of the New York-based law firm Stroock & Stroock & Lavan. “I was miserable, working these really long days, and my father-in-law was getting older; Steven needed some help,” Mr. Berlin said. In 1990, Mr. Berlin joined the firm, and soon found his niche in curtains, a far cry from legal insurance work, and a world in itself with its valances and swags and jabots. He learned on the job, and now creates sumptuous curtain patterns and supervises their production. “My wife calls me MacGyver. You can put me in a situation with chewing gum and twigs and I can make a house.”

They’re rarely called upon for such derring-do, but Messrs. Jonas and Berlin offer a more staid but useful service to their clients, generally architects and designers: the ability to build furniture to someone’s precise specifications. “We can make a chair softer, the back straighter, add more detailing, anything you like,” Mr. Berlin said. After a sample is built, a client often visits the showroom and tests out the furniture, suggesting alterations. (Chairs run about $4,500 and sofas start at $10,000.)

In this vein, Jonas Interiors has built up an enviable clientele through the decades. Mr. Jonas ticked off names: “Two White Houses, the Saudi royal family, the Gettys, the Bronfmans, the Rothschilds, the Spielbergs, the Geffens. Ralph Lauren; I did all his furniture and curtains in his Bedford house, and he’s in the furniture business,” he added, laughing.

Mr. Berlin told of flying back and forth to Carl Icahn’s property in Las Vegas several times until the decorator was satisfied with the curtain trim. Mr. Jonas spoke of a job they did at the Gardiners estate on Gardiners Island, the private island off of the Hamptons whose owner recently died. “The ferries weren’t running so we had guys who had to live out there for two weeks, working on installation.”

But such high demands are par for the course. “We built a chair for a very well-known client in the financial industry,” said Mr. Jonas, uncharacteristically deciding not to name names, “and he liked the chair so much that he ended up ordering 40 versions of the same chair, to be placed in his various homes. ‘I want to be comfortable where I go,’ he’d say.”

And for Mr. Jonas, a man who appreciates brass tacks, that’s all that matters. “Like my father used to say, ‘The biggest compliment is a purchase.’ “