Jethro Tull's second album, Stand Up, was transitional for the band; founding guitarist Mick Abrahams left the group, and with him went a good portion of the blues-rock flavor that defined Tull's first album. Abrahams' replacement, Martin Barre, would become the second most important contributor to the band's sound after frontman Ian Anderson. Barre knew his way around a hard-rocking riff too, as evidenced by the likes of "A New Day Yesterday" and "Nothing Is Easy," but he could also follow Anderson and company in subtler directions, pointing toward the path Tull would take on their best-known albums. The folk influence that proved crucial to the band's subsequent work can be heard on acoustic-oriented tunes like "Look into the Sun" and "Reason for Waiting." The first flowerings of Tull's world-music influence is found in the exotic-sounding "Fat Man," while the classical element that would fuel their future prog-rock excursions appears here in the form of the instrumental track "Bouree," a flute feature based on Bach's Bourrée in E Minor.