Until 1987, Philadelphians observed a polite understanding that no building should rise above Billy Penn, the bronze statue keeping watch atop City Hall at 548 feet. Then along came Liberty Place.

Developer Willard Rouse proposed twin towers of 61 stories and 960 feet, and because the tradition rested on agreement rather than ordinance, there was nothing to stop him. Rouse reportedly told his architect, Helmut Jahn, that he had better design something spectacular, or Philadelphia would never permit another tall building. Jahn obliged. The result, which deliberately echoes Manhattan's Chrysler Building, transformed the skyline overnight.

VoiceMap's self-guided audio tours use Liberty Place to mark the moment Philadelphia's skyline broke open, connecting the towers to Billy Penn on City Hall and to the cluster of taller buildings that followed once the unwritten height limit fell.