Two duos – and one unique instrument – create a spark.

For 18 years, fiddler John Weed and guitarist Stuart Mason have populated two-thirds of the local fave Celtic trio Molly’s Revenge. This weekend they play Pacific Grove as a quartet called New World String Project, part of a California coastal tour.

Weed and Mason have hooked up with two new players, Windham Hill harpist/cittern player Lisa Lynne and multi-instrumentalist Aryeh Frankfurter, who plays one of the most unusual instruments you’ll ever hear.

It’s called the nyckelharpa in Sweden. The modern version is a 16-string instrument with three melody strings, one drone string and 12 sympathetic strings that pick up sonorities from the three melody strings, not unlike the drone strings of the East Indian sitar.

Got all that? Well, there’s more.

Thirty-seven wooden keys are arranged to slide under the strings, performing the same function as the frets on a guitar – to change the pitch depending upon which key is deployed.

The player deploys a violin-like bow on the melody strings with the right hand, and pushes on the keys with the left. It sounds something like a fiddle, only with more resonance and depth, like the sound of playing in a large cathedral.

The instrument and Frankfurter’s touch transform Weed and Mason’s music.

“We played with Aryeh and Lisa at a house concert John and I did, and the crowd went crazy,” Mason says. “There was a standing ovation, and we knew we had a spark there.”

Frankfurter and Lynne are an established duo, as are Weed and Mason, so the collaboration is more powerful than starting a quartet from scratch.

NEW WORLD STRING PROJECT 3pm Sunday, May 6. St. Mary’s by-the-Sea Episcopal Church, 146 12th St., Pacific Grove. $22/at the door; $20/advance; $10/children at the door; $8/children in advance. 373-4441, stmarysbythesea.org.