DULUTH — Once a month, typically on a Sunday afternoon, the Friends Meeting House is filled with voices singing together. It's not a performance because everyone present is involved in the singing.

The crowded room echoes with lilting voices raised in a simple, timeless song. There are no instruments, no audience. Just a chorus of four-part harmonies sung from a book that's as thick as a Bible.

NPR's Scott Simon speaks with Laura Atkinson and Justin Hicks of Louisville Public Media about shape note singing and its influence across the American musical tradition. Shape note singing is one of ...