Every week, Norumbega Harmony sings hymns together, including on the second Monday of the month at Church of the Redeemer in Chestnut Hill. But it’s a little different from what you might expect.

If you’ve been to a church, whether that church is Methodist or Catholic or Unitarian, you’ve seen a hymnal: a book about 9-by-6 inches with musical notation that looks like the music you’d find in a secular context.

But before this became common, there was another Christian singing tradition in New England: shape note singing, also known as ‘Sacred Harp’ after its primary songbook. It’s an a cappella style of singing that is intended for all to be able to participate, not solely being led by a choir.

“There isn’t a director picking the music,” said Sylvia Martin, a shape note singer for over 30 years who has also been in traditional church choirs. “People come and pick a tune, and we sing it once, and then someone else picks a tune, and it goes on that way throughout the evening. It’s very informal and democratic. The camaraderie of the people, the warmth and friendship of the group is the best part.”

Its books are much wider than six inches, and its musical notation has notes with different shapes, hence the name. It arose in a time when churches were moving away from congregants memorizing hymns but before musical notation was fully standardized.

William Billings published the New England Psalm-Book in 1770, and today this forms a large part of the musical tradition. Congregations would often have a singing master visit for a few weeks to teach their congregants the new style.

The tradition faded out in New England by the early 19th century because clergy wanted simpler music that was easier to sing.

“When Billings began setting songs to music, it started out straightforward, but as the form grew, the composers began to branch out,” said Martin. “There are voices coming in at different parts. There are anthems, which are long, prose texts from Scripture. They can be difficult and have bigger leaps between notes. It got elaborate.”

This led to the adoption of the European-style hymnal that is common today. But shape note singing was kept alive in the South, and in the 1970s, Wellesley College professor Stephen Marini visited Southern churches to learn from them.

Out of that, Norumbega Harmony was born.

Today, there are shape note singings all over the country and the world. The next edition of the Sacred Harp book will include tunes from composers who are not American, reflecting the new international interest in the tradition.

But it’s an important part of the New England heritage in particular.

“A town or a congregation that has been around since the 1700s will invite us to come and sing,” said Martin.

On March 30 at 3 p.m., they’ll be at Newton Highlands Congregational Church, singing colonial-era music for the 250th anniversary of the beginning of the Revolutionary War.

They don’t dress up, though.

“In my opinion, such costuming increases the performative distance between singers and hearers instead of mediating it,” said Marini in Commonplace, a journal about early American life.

Marini will also be speaking about the tradition at the March 30 event.

The group has been at Church of the Redeemer on the second Monday of the month from 7:30 to 9:15 p.m. since the closure of Andover Newton Theological School. But they’re not affiliated with a particular denomination, and everyone is welcome to come and sing.

Although church choirs will occasionally do shape note singing as part of a demonstration of historic church music, it’s no longer a regular aspect of a worship service in New England churches. Some of the music is shared with the more commonly used European-style hymn tradition, like Amazing Grace, but not all of it. One aspect of shape-note singing that’s different for many is that the melody is on the tenor line, not the soprano line.

“Once people get familiar with the songs, they love it and keep coming back,” said Martin.

More details can be found on Boston Sacred Harp. If you’d just like to hear the music, in addition to the March 30 event at Newton Highlands Congregational, you can also visit the Old Stoughton Music Society in Stoughton, which was founded by Billings and is the oldest continuously operating choral society in the U.S.