On Seven Locks Road, just south of the Montgomery County detention center, stands a rustic wooden signboard that announces in fancy green lettering the coming of a new development on that site called Potomac Springs.

Drive a few yards further south on Seven Locks and there's another sign that says "Welcome to Rockville."

Potomac Springs, despite its name, is in Rockville and not in well-known, exclusive Potomac. In fact, it is six miles away from the intersection of Falls and River Roads, which is the heart of Potomac.

However, since the name Potomac is surban real estate is what Tiffany is to jewelry and Steinway is to pianos, a number of developers have found it extremely rewarding to use the name in advertising developments sprouting up on the outskirts of Rockville and - in the case of Potomac Springs - right within the Rockville city limits.

The name Potomac, says Porter Wilson, a sales manager for Burger Berman, the Potomac Springs developer, "speaks for itself . . . it sounds much more appealing than 'Rockville' Springs. It's a drawing card.'

Developers say the name has an "abracadebra" effect on some prospective home buyers who are impressed by the mystique of Potomac.

Traditionally, Potomac has represented a haven for the wealthy who live in custom-built homes nestled in the countryside behind white clapboard fences that are worth in some cases between $300,000 to $500,000.

Even today Potomac in still known as horse country with five to 25-acre estates, a well-known hunt club and country club - a place, to borrow a phrase from F. Scott Fitzgerald, where "people play polo and are rich together."

Around the edges of what is traditionally known as Potomac are sprouting developments that boast "Potomac" somewhere in their names> yet look curiously like high density developments found in less exclusive areas of suburban Maryland.

"Potomac for under $80,000" reads one ad for the Potomac Meadows development, which is on Richie Parkway in Rockville.

Even developments on the outskirts of Gaithersburg are using the Potomac name. One such development, Potomac Chase Estates, is advertised as a choice location "where Potomac hunt country ends."

While the name Potomac represents prestige to buyers, it means added dollars for builders and developers, who say they can automatically add several thousand dollars to the price of the homes in Potomac and still be certain of getting a willing buyer.

A builder who asked not to be named said the same homes - same acreage, same floor plan - that he built in Rockville and sold for $80,000, sell for about $20,000 more in Potomac.

Its a common practice and perfectly legal.

Potomac, since it is not an incorporated town, has no official city limits, though its heart is recognized as the intersection of Falls and River Roads.

Even Maryland Park and Planning Commission boundaries for Potomac are general. The Commission says it extends anywhere from I-270 on the east to the Potomac River on the west and from the Rockville city line on the north to the borders of Bethesda on the south.

Parts of Rockville share with Potomac the same postal zip code - 20854. So residents who live within that zip code boundary can list their mailing addresses Rockville or Potomac, depending on their preferences.

What comes down to is that people are willing to pay higher prices or the Potomac mailing address.

"It's not necessarily that builders are making more money (on Potomac homes)," said builder John Kossow. "It's just that land costs so much more in Potomac."

The land is worth more because demand for it is high. An acre may cost as much as $50,000, developers say. But the prices don't seem to deter those who hunger for a Potomac address.

"People want to step up to Potomac. They consider it the top in Montgomery County. Maybe even the state," said Wilson of the building firm called Berger Berman.

Old Potomac residents, however, look with chagrin at the new high density developments invading the countryside.

"I absolutely think it's terrible. It isn't Potomac with all these houses and people. The new houses don't even have white clapboard fences," said Lois Eiseninger, a Potomac resident for 20 years.

"Some (of the new developments) are very nice homes, but by Potomac standards they are right on top of each other. It's quite a change from 10 or so years ago. Potomac homes were always two or more acres," says 17-year Potomac resident Art Johnson, former president of the West Montgomery County Citizens Association.

"They can't afford the mansions, but they want to be near them," said one Potomac builder.

People who live in the newer subdivisions say they feel as much a part of Potomac as the older more established residents.

"Potomac in your eyes might be houses on a five-acre plot of land with horses running around," said Robert Benson, a Montgomery Square resident who lists Potomac as his address. "But there are a lot of people with homes that cost around $100,000 who would be mad if you say they're not in Potomac.

Montgomery Square, built by Kettler Brothers, was one of the first high density developments on the Rockville-Potomac borderline.

Most of the development's residents use a Potomac mailing address, according to Sam Barrow of Kettler Brothers, who also lives in Montgomery Square.

"I recall an instance when two people who were living in Montgomery Square got together and one party was telling the other about this lively new house they had just bought in Potomac, while the others were talking about the new house they had just purchased in Rockville. As it turned out, they lived right around the corner from one another."

Buyers, however, are well aware of where they are buying if it is in Rockville, developers say. Since county law requires sales people to show a copy of the Rockville city master plan to anyone who purchases a home in Rockville.

"I've never heard of anyone being unsavry enough to be duped," said Nancy Goldstein who is in charge of Potomac sales for the Colquitt Corruthers real estate firm.

Still, Ruth Rhodes, a switchboard operator at the Rockville post office, says she receives calls from irate citizens who want to know why the post Office permits mail to be delivered to their "Potomac" homes when it "incorrectly" lists their address as in Rockville.