During the Gilded Age, Rittenhouse Square was home to Philadelphia's high society, with more millionaires per square foot than any other American neighborhood except New York's Fifth Avenue. Established by William Penn in 1682 as the South-West Square and renamed after astronomer David Rittenhouse in 1825, Rittenhouse Square and its environs changed from an isolated district of brickyards and workers' shanties into the city's most elegant and elite neighborhood between 1845 and 1865. The brownstone and marble mansions on the square itself were inhabited by the city's wealthiest and most prestigious families, with names like Biddle, Cassatt, Drexel, Stotesbury, and Van Rensselaer. As Philadelphia's upper classes fled to the suburbs in the early 20th century, their mansions were replaced by skyscrapers or taken over by cultural institutions like the Philadelphia Art Alliance and the Curtis Institute of Music. While only a few original residences remain on Rittenhouse Square, it is still the center of a lively upscale neighborhood.