It's been 100 years since Philadelphia sliced the grand diagonal of the Benjamin Franklin Parkway through the city's regimented, right-angled grid. The new boulevard helped bring the lush natural landscape of Fairmount Park into the heart of the city, but it also left the city with a string of triangular blocks that were tricky to develop.

You might expect architects to respond to the challenge by designing triangular buildings on triangular blocks — variations of New York's three-sided Flatiron building. Instead, they chose a different strategy. Between 16th and 18th Streets, you'll find three circular, or nearly circular, structures, all products of mid-century playfulness : Embassy Suites, Windsor Suites, and the LOVE Park Visitor Center.

There are several possible motivations for putting a circle in a triangle. Because the Parkway was inspired by City Beautiful planning principles and beaux-arts design,  a round building can be seen as the architectural equivalent of a classical column. Like lighthouses at the tip of a peninsula, the cylindrical shape helps focus the eye toward an end point. Round buildings serve as an exclamation mark, punctuating the corner and drawing people from multiple directions. For developers interested in leasing space, the Parkway buildings also promised wide-angle views of the city.

The Visitor Center at 16th and JFK Boulevard was the first of the three cylindrical structures on the Parkway. Designed by architect Roy Larson and engineer Nicholas Gianopulas, it was built when the Parkway still bisected the block that is now home to LOVE Park, breaking it into two triangular parcels.

By the time the Visitor Center opened in 1960, the rage for circular structures was in full swing. The curvy aesthetic, which gave us Bertrand Goldberg's Marina City in Chicago and numerous small banks, was part of the growing revolt against the straight lines and strict functionalism demanded by orthodox modernists, says William Whitaker, curator of Penn's Architectural Archives. Within a year of the Visitor Center's opening, two more round buildings had been commissioned for the Parkway. Both were completed by 1965.

Technically speaking, neither the Windsor nor the Embassy Suites is a true circle. The 25-story Windsor, designed by Aaron Colish, a student of Paul Cret, uses its broad, curving facade to unite three streets, 17th, Cherry, and the Parkway, but includes a rectangular wing in the back. Having once spent two months as a resident of the Windsor, I can confirm that the apartments are shaped like a wedge of pie and culminate in a terrace.

Embassy Suites, designed by Oskar Stonorov for the Korman Co., started out as the Plaza Apartments. Though the 27-story building at the corner of 18th Street gives the impression of being a sleek cylinder, it is assembled from 12 flat planes. Pairs of recessed balconies alternate with white marble panels. Using flat sections to create the impression of roundness was probably easier and cheaper than fashioning a real curved surface. But like its curvy neighbors, it clearly makes its point on the city skyline.