Tanya Donelly and Chris Brokaw are going medieval on listeners

They were not going to sing “I Saw Mommy Kissing Santa Claus.”

That was the pact that Tanya Donelly and Chris Brokaw made when they decided to do a set at the annual holiday show organized by their fellow longtime Boston rockers Hilken Mancini and Melissa Gibbs.

Looking for something different to sing that night at The Burren, Brokaw thought about a record he’d been listening to by the Norwegian group Trio Mediæval. “I thought, ‘What if we did a medieval Christmas carol?’” recalled Brokaw. It worked so well that the two found three additional medieval songs to rearrange for Donelly’s voice and Brokaw’s electric guitar. They’ve collected them on their new EP, “The Undone is Done Again,” and will feature them as part of a tour. Donnelly and Brokaw perform Friday, May 1, at First Church in Cambridge Congregational for Global Arts Live and on Saturday, May 2, at The Town and The City Festival in Lowell.

Both have been major figures in Boston rock for decades: Donelly with Belly, Throwing Muses, The Breeders and her solo output; and Brokaw with Come, Codeine and numerous other projects. But surprisingly, they barely knew each other. “We had a million friends in common, but I don't think we really had any conversations together before this,” said Brokaw.

Donelly spotted Brokaw’s name on a mass e-mail that had gone out to the different artists who’d been invited to appear at the 2024 holiday benefit. “And then I ran into him at a Gang of Four show in Gloucester at The Cut,” she recalled. “And I did something very out of character for me: I just waltzed up to him and asked if I could crash his set at the shindig. Maybe sing backup harmonies, or we’d combine our sets, or do a Christmas carol.”

Brokaw liked the idea of a carol, but the one he had in mind was “In hoc anni circulo,” a telling of the story of the Virgin Mary believed to have originated in Germany around the 12th century. Heard through the lens of Donelly’s ethereal singing and Brokaw’s spare, haunting guitar, it’s both haunting and comforting.

“People seemed really struck by it, and I was like, man, I think we should do more of this,” said Brokaw. “Tanya agreed, and we picked another three songs, and that felt like a complete statement.” The project is being released by Fire Records, a U.K.-based label that Brokaw as well as Donelly’s sister, Kristin Hersh, have recorded for in the past.

“I love the give and take,” said Donelly of recording the medieval ballads and carols with Brokaw. “I use the word ‘trancy’ a lot to describe it. I went to this other place, and it was really kind of magical and wonderful.”

While diving into medieval music for source material, several friends of Brokaw suggested he listen to Anonymous 4, a popular vocal group that actively recorded and performed concerts of medieval music between 1992 and 2016. Brokaw bought a boxed set of their recordings, which included an album devoted to carols. It included an acapella piece, "Plaudat Letitia." Brokaw was inspired to come up with a guitar part for his version with Donelly. “When we do it live, it’s really free form,” he said. “It keeps us on our toes,” added Donelly.

For Donelly, there was another challenge: The songs are in Latin, which she readily admits she doesn’t speak. To help, she got translations of the songs before she recorded them. “I wanted to make sure that it was something that I felt comfortable singing,” she said. “And some of them have many, many verses, so I went through and picked out the verses that appealed to me. I wanted to make sure that subjects were things I could get behind, and also just how they sounded coming out of my mouth.”

The material also gives Donelly the chance to showcase a part of her vocal talents that aren’t usually in the forefront when she’s rocking out with Belly.

“It is a territory I've sort of played with before, but mostly doing airy backups on things,” she said. “A lot of it is coming from a different space in my body that I’ve never used before.”

For their live shows, the duo is singing the medieval pieces as well as new arrangements of songs that each of them has recorded over the years. They’re also debuting some new material that they’ve written for their next project, which they’re recording next month with Dave Minehan, who recorded “The Undone is Done Again.”

“Now we’re writing some original songs that are somewhat in the same vein sonically,” said Donelly. There is one difference: The songs are in English, not Latin.