Numbered protractors have been found on mailboxes, telephone boxes and other locations.
The mathematical instruments, which are most commonly used to measure angles, are securely affixed to the chosen items. Some people are amused by the appearance of the protractors, but the city's graffiti task force said the person responsible could face charges if caught.
“It could either be a renegade math freak or someone who just loves intrigue,” Jim Mueller, a resident of one of the areas where the instruments have appeared, suggested to CBS
It appears that the protractors began appearing a while ago.
"I've been seeing these for months," commented nochasingiguanas on the Bike PGH
website three weeks ago. "I would mention them to people, and no one knew what I was talking about. I'd love to know what this project is is about."
Eric Lidji, 28, was documenting the locations of the protractors.
"It taps into childhood feelings of collecting things, memories of scavenger hunts," he told the Pittsbugh Tribune-Review.
"It's like a pen pal or a chat room."
He had documented about 80, and posted the information on his blog, until he heard that charges could be involved.
"The protractors initially caught my interest because searching for them felt like stumbling onto a charming citywide mystery," he wrote on his blog
. "But I have no interest in getting anyone in trouble. So the project is on pause for now, at least publicly.
"I imagine I'll still compile the list away from the glare of the Internet."
Pittsburgh police Detective Alphonso Sloan of the city's graffiti task force told the Pittsburgh Tribune-Review
"Considering (whoever is responsible) is defacing public property, it could definitely be at least criminal mischief," he said. "We're going to look into it now that we know about it."
Some photos show protractors stuck to surfaces where there is so much graffiti that there is barely any bare space.
Video reports on the protractors can be seen here