Taking influence from the popular style of 40s and 50s rock & roll, as well as electric blues, blues rock adopted a heavier sound. By the mid-60s, bands like Cream and The Yardbirds had incorporated blues fundamentals, such as the common 12-bar blues chord progression and extended improvised guitar solos, into their more riff-based song structures. Following rock tradition, bands were comprised of loud, amplified guitars and bass, as well as drums. An important progenitor of the style is John Mayall who significantly developed the sound of blues guitar and, in 1966, released the album Blues Breakers with Eric Clapton which defined the previously ambiguous genre.