Dolly Parton, Emmylou Harris and Linda Ronstadt

Johnnie Walker talks in depth to three of America's greatest singer-songwriters, who between them have sold more than 200 million records, won at least 20 Grammys and can sell out concert halls and arenas in moments.

Emmylou Harris proclaimed "it was a glorious sound" when she first sang with her friend Linda Ronstadt and their "favourite" singer Dolly Parton in 1975 at her house in LA. As you will hear, the harmonies blended as if they had been singing together from birth, creating what they called "the sisterhood of song". Dolly says just hearing the subsequent recordings still gives her chills. Many believe that they created what we now call Americana, with the Trio project.

Johnnie hears about those first meetings and the struggle to get their schedules aligned for two albums (released twelve years apart), plus the fun they had choosing the songs and who would sing them, working in the studio with some amazing musicians and winning awards and critical and audience acclaim, plus their thoughts on the other artists they covered - including Linda Thompson, Randy Newman, the Carter Family and Neil Young.

Trio is the perfect description for their equal and delicately balanced collaboration, as the albums remain their favourite works and are, in their own words, their enduring legacy.