Staying power in retail is vanishingly rare.

Then again, there's Sophy Curson on posh Rittenhouse Square. It just celebrated its 87th birthday last month as the city's most venerable women's fashion boutique.

It's seen quite a bit: the Great Depression, World War II, the rise of hot pants and miniskirts (1960s), bell bottoms (1970s), shoulder pads (1980s), the Jackie Onassis-inspired "minimal look" (1990s), animal prints (2000-10), the current comeback of lace. It's also seen the decline of brick- and-mortar retail stores.

"We provide extraordinary service and exclusivity that you can't find anywhere else," said David Schwartz, Sophy Curson's great-nephew. He runs the store with his mom, Susan, 79, whose aunt, the store's namesake, died in 2004 at 101.

Sophy Curson is known for its selection of special-occasion fashions and little black dresses and accessories. It's that go-to place when you need a gown for the Academy Ball.

"We get a lot of tourists and they love it, because when you go the mall, it's just one designer in one boutique," said David. "Here, we have [dozens] of different designers."

Customers "are not used to having one-on-one," Susan said. "Because we know how clothing fits and the dimensions of people, they're not wasting their time trying on things that don't fit. And they get used to that."

Seamstresses work in the basement so dresses and gowns can be altered on-site and changes can be made to some designer dresses.

"We buy what we love," said David, 50. "We try to buy something that may be classic, but we pair it with something that is unexpected."

Every piece of merchandise and jewelry is hand-selected by mother and son, who travel to Europe twice a year to buy. Last month, they were in New York for Fashion Week.

Sophy Curson sits on some of the city's most prized real estate. Being housed in its own building helps, say brokers, and that's why it hasn't been at the mercy of skyrocketing rents due to demand of late.

"Rittenhouse Square is the crown jewel of Philadelphia real estate," said Jacob Cooper at MSC Retail Inc. "While every single neighborhood surrounding Center City is undergoing a renaissance right now, Rittenhouse Square is the hall of fame for retail and restaurants.

"What drives value? It is the seven-day-a-week pedestrian flow around the square," he said. "The intersection of residential, office, hotel, and retail traffic creates a vibrancy that is unmatched in the city. Retailers, like Sophy Curson, who own their real estate have absolutely benefited from the resurgence of the square in the past 25 years. They lived through the bad times and now can enjoy the great times."

In July 2010, the vacancy rate for Rittenhouse Square was about 12 percent, say commercial brokers. This year, it is at 6.7 percent.

Likewise, rents along Walnut Street in July 2010 averaged $105 per square foot; they're now averaging about $150 per foot. Street corners have exceeded $200 per square foot.

Larry Steinberg, executive vice president at CBRE Inc., said that over the last few years there has been a transition away from luxury retailers such as Burberry and Coach.

"Sophy Curson has defied the recent exit of luxury retail, providing a much-needed fashion resource for the empty-nesters that continue to have a significant presence in Rittenhouse Square," he said.

Steinberg said the boutique has kept its cache even amid the recent surge of millennials in Center City that has attracted a wave of retailers such as Under Armour, Warby Parker, Bonobos, and Vince.

On a recent Saturday, a long, flowing fuscia gown in a silk gauze with a peekaboo cut on the bottom by the Canadian designer Wayne Clark was showcased on a raised stage in Sophy's storefront display.

Inside the narrow, 1,000-square-foot store on the corner of 19th and Sansom Streets were racks of clothes on one side and fitting rooms on the other, each room with several pairs of high-heel shoes so "customers could try on a dress with the appropriate shoes," said David.

New this fall are outfits by the Italian sportswear designer Fabiana Filippi; the husband-and-wife team Tom and Linda Platt, who produce a modern line of daytime and evening fashions; the French brand St. James, famous for the Navy-striped shirt; and the American designer Planet, with asymmetrical jackets (one retailed for $300 with matching pants, also $300).

A Catherine Regehr magenta party dress with Swarovski crystals had a $2,400 price tag - the kind of fancy evening wear that's been Sophy Curson's signature.

"If you tell us which event you are attending, we keep a list and make certain no doubles are sold," David boasted. "Our customers rely on us for this practice. This is hugely important to them, so it is hugely important to us."

Sophy Curson first opened her boutique the year the stock market crashed in 1929. The reason was personal.

"Aunt Sophy started the store because she couldn't find any clothes that were made for her stature," Susan Schwartz said. "She was 5-foot-1. That was exclusively the store until the 1960s" - clothes for petite women.

David joined the company in 1988, the summer after he graduated from Kenyon College. He'd had plans to head to Hollywood.

But fate intervened.

"I was doing merchandising and went on a buying trip with my mother to Europe, and when I came back, I got into a groove," he said.

Some 27 years later, he's still doing what he loves.

"We have evolved over the years but still take great pride in continuing to provide the attention to detail and unmatched service that was so important to Aunt Sophy," David said. The goal then and now was this: "You shine in the dress and the dress isn't wearing you."

215-854-4184@SuzParmley