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.