In its purest form, Rock & Roll has three chords, a strong, insistent back beat, and a catchy melody. Early rock & roll drew from a variety of sources, primarily blues, R&B, and country, but also gospel, traditional pop, jazz, and folk. All of these influences combined in a simple, blues-based song structure that was fast, danceable, and catchy. The first wave of rock & rollers -- Chuck Berry, Elvis Presley, Little Richard, Jerry Lee Lewis, Buddy Holly, Bo Diddley, Bill Haley, Gene Vincent, the Everly Brothers, and Carl Perkins, among many others -- set the template for rock & roll that was followed over the next four decades.