Johnnie & Jack mined the familiar turf of singing brother duos in the late '40s through the late '50s with a few distinct twists. For openers, they weren't blood brothers, just brothers in law. Secondly, they brought a new rhythmic strain to country music, both in their use of Latin beats and the unfettered drive of their combo, the Tennessee Mountain Boys. And of all the singing duos, they were the most inclined to stretch the boundaries of their sound, from bluegrass to sacred to amazing covers of R&B tunes with none of their country-soul diluted in the bargain. But for all their melding of outside influences, few artists -- even in the mid-'50s -- were as wholesale committed to sounding as "country" as they were. Whatever they played, sang, or wrote, it always sounded like Johnnie & Jack.