Ira and Charlie Louvin sang harmony so close and so charged that other musicians spoke of it with something like awe. Ira's high tenor and mandolin against Charlie's steady lead and guitar produced a sound — gorgeous, eerie, a little frightening — that influenced everyone from the Everly Brothers to Gram Parsons, and that bluegrass bands have been chasing ever since.

The brothers came out of north Alabama and recorded gospel sides for a few labels before signing, after a brief stop at MGM, with Capitol Records in 1952. Capitol producer Ken Nelson kept them at first on the sacred material they loved, and their gospel singles and the album Satan Is Real became touchstones of country religious music. When commercial pressure pushed them toward secular songs, they answered with "When I Stop Dreaming," "I Don't Believe You've Met My Baby," and a string of others that proved the harmony worked just as well on heartbreak as on hymns.

The Louvins were not a bluegrass band — their records used a country rhythm section, and Ira's mandolin was a harmony and texture instrument rather than a lead-driving one. But their material and, above all, their harmony singing went straight into the bluegrass bloodstream. The duet style, the gospel quartets, the particular ache of two brothers a third apart: bluegrass harmony singers study the Louvin Brothers the way a painter studies a master.

Ira's drinking broke the act up in 1963, and he died in a car crash two years later. The singles remain a high-water mark for harmony singing in any American style.

Session details drawn in part from the Bluegrass Discography.