Pelli, a former Yale Architecture dean, currently holds the title of designer of the world's tallest buildings, the Petronas Towers in Kuala Lumpur, Malaysia. Pelli is known for his use of simple geometric forms, such as the domes and pyramids that top his World Financial Center complex in New York.

Stern's Philadelphia building will rise 52 stories above Suburban Station at 17th and JFK and feature a winter garden on top and a tree-filled atrium at street level. The architect has chosen to use the same warm beige stone used to build the Philadelphia Museum of Art. Stern says it's a "nice gesture to the building on the Parkway [without] trying to pretend to be looking like that building." Drawing "lessons" from the PSFS building at 12th and Market, America's first modernist tower, which Stern calls "one of the best skyscrapers ever built," he is "trying to make a quiet tower, a simple tower."

The irregularly angled prism-shaped 28-story Pelli building is to be built next to 30th Street Station. The semi-reflective surface will pick up the sky, the river and the greenery of Fairmount Park. The location will allow for easy transportation access by train and car and provide tax breaks for the developer. The area just west of the Schuylkill is a Keystone Opportunity Zone, part of a state program to encourage development in private areas.

Mark Shoemaker, who led the design team at Pelli and Associates, understood the aesthetic importance of the location. "This building, by virtue of where it was, was going to have an impact on the station and an even bigger impact on the skyline of Philadelphia. Even at [roughly] 30 stories it was going to stand out and end up anchoring the western end of that skyline." Shoemaker says Pelli aimed to design a "striking [building] worthy of that skyline position."

But in the current economic climate, will either of these buildings actually get built? Even Stern himself wouldn't weigh in: "I've learned that I can talk about architecture, not real estate."

According to the most recent report on the Center City office-space market, there is already a lot of vacant space. For top-of-the-line "Class A" space, the vacancy rate has grown from 6.85 percent to 10.65 percent over the past three years.

Observers of the local real estate scene say announcements about fancy new towers going up are advertisements for prospective tenants. If tenants show interest, the building will go up; if not, the towers will never exist anywhere but in the architect's head.

Liberty Property Trust, which is developing the Stern building, won't begin construction until it has secured an anchor tenant. "We won't start a building anywhere that's 100 percent speculative," says John Gattuso, senior vice president for Urban Development and National Marketing at Liberty.

Initially, Liberty president Willard Rouse said he would break ground in August. Now the company is giving itself some leeway, saying only that it will begin construction in the third quarter of this year.

Brandywine Realty president Jerry Sweeney says his firm is aiming to complete the Pelli building in 2005 "when we feel the market will be in a stronger position." As for the fact that downtown office space in Philadelphia costs far less than in other East Coast cities with comparable vacancy rates, Sweeney replies, "What a wonderful way to bring in people from outside the area." Even if rents in the tower are pricey by Philadelphia standards, they'll be a bargain compared to New York or Boston.

Gattuso isn't banking on tenants moving in from beyond the region. "That's typically not the driving force for office growth," he says. "Growth of our indigenous companies is."