“Could it be another rogue fairy?” Theo asked.

“Is it possible? Yes. But what’s the benefit to fae of sending Cadogan House somewhere else? That’s just negative attention on the entire group, with no obvious benefit. Claudia didn’t think a demon would need an accomplice. But we do have a bigger problem.”

I told them about the time limit.

“Two seventeen a.m. on the nineteenth,” Petra said, looking at her screen. “That’s the exact time of the full moon.”

“So we’ve got, like, three days,” Theo said, then blew out a breath.

“Yeah,” I said. “So let’s get on it?”

“Our best bet to get Cadogan House back is to get the demon,” Petra said. “And our best bet to get the demon is to find the Cornerstones first and trap her when she tries to get one.” She put an image on the overhead screen. “Here’s the map we discussed.”

We all looked at it in silence: the wards, the Cornerstones, and the ley lines.

“The warehouse Cornerstone is pretty close to the first ley line,” I said, “but South Gate’s nowhere near one. So that’s one theory out.”

“That’s good news,” Petra said. “We know the Cornerstones aren’t literally on the ley lines. Think of all the points we don’t have to search now.”

“It’s probably another defensive consideration,” Theo said. “Ley lines are the first place anyone would search.” He gestured to all of us. “Case in point.”

“Unfortunately, it doesn’t give us any better idea of where the Cornerstones actually are and why.” I looked at Petra. “What about Cadogan House?”

“Gwen didn’t find anything in the property records,” Petra said.

“We only know of two Cornerstone locations,” I said, “but I’m not seeing anything that suggests there’s some obvious pattern. Like we discussed, if the Guardians wanted to build concentric defense rings, these positions don’t make sense.”

Theo nodded. “There’d have to be an intermediate ring, but she didn’t trigger one.”

“What about neighborhoods?” I asked.

“Neighborhoods?” Theo asked, frowning.

“Like, community areas or political wards? Maybe it was one ward per area? Can you get a map of legislative districts from 1872? Add a layer for that?”

“Hmm,” Petra said, but she was already looking. It took less than a minute for her to find a map, and less than that for her to superimpose it over the ward locations.

“There were twenty political wards in Chicago in 1872,” Theo said. “That leaves eighteen more potential Cornerstones, if they used those boundaries as a guide.”

“That’s too many,” I said, scanning the map. “I don’t think they have the magical resources for that. Not without sorcerers.”

Petra narrowed her gaze in what I’d decided was her Thinking Stare. “Maybe math can help.”

“Math?” I asked.

“An algorithm. We calculate the distances and angles between the Cornerstones and ley lines that we know about, and we predict where the other ones might be. I’ve got a cousin who’s a mathematician.”

“Do you have any below-average cousins?” Theo asked. “Who aren’t mathematicians or doctors with generously portioned wedding parties?”