Architect Robert Venturi got a tricky assignment in 1976: The National Park Service wanted to commemorate Ben Franklin’s residence in Philadelphia, but Franklin’s house had been demolished in 1812 and no record of its appearance had survived.

“There were fire insurance descriptions and archaeological remains of the house, so you could tell the exact configuration of the walls, and there were letters about the house exchanged between Franklin and his wife while he was in London and the house was in construction,” Venturi said. “But, with so little visual information, the Park Service was pretty easily persuaded not to try to reproduce the house.”

He solved the problem with a “ghost house” outlining the dimensions of the original house with a steel armature. “The aim was to create a delightful open place in the center of the dense texture of the city,” he said. “So the courtyard became a pleasant neighborhood amenity for people who live there, as well as for tourists.”