Hidden in plain sight around Pittsburgh are hundreds of numbered protractors glued to various surfaces. Their purpose—and provenance—remains a mystery of the first order. 

The geometry implements are securely fastened to bridges, garbage cans, stones, mailboxes, benches, street signs, and other objects around the city. They can be found in many neighborhoods, but the Polish Hill neighborhood has one of the largest concentrations.

Each is assigned its own number, and the numbers run into the hundreds. Reports of protractors began coming out of the ‘Burgh, as it’s colloquially known, in 2011. The reason for the protractors isn’t readily apparent, leading to manifold interpretations.

Some think they’re a celebration of the steel city, reminiscent of the city’s famous bridges. Others postulate that the tools were plastered everywhere as part of an elaborate art project or the work of a lone “renegade math freak.” Still others believe the protractors have been placed to encourage Pittsburghers to uncover the hidden splendors within their city.

Some don’t find the phenomenon as intriguing. The protractors have attracted the attention of the authorities, who say they’re akin to graffiti and a crime. By all accounts, the tools are super glued to each surface and would prove difficult and expensive for the city to remove.  

Many online have traded theories, photos, and accounts of their encounters with the plastic half-circles. Some protractor devotees have plotted their locations on maps, adding locations as they are discovered, connecting the dots, trying to divine their meaning.