This document discusses the evolution of William Penn's original grid plan for Philadelphia over three centuries. Penn based the grid on plans for rebuilding London, with squares meant for public use similar to London's Moorfields. In the 19th century, most squares became public parks. In the 20th century, traffic needs led two squares to be reconfigured as circles. The plan shows how Penn's original grid endured but the uses and meanings of the squares changed over time to accommodate new transportation and recreation needs.