The modal theory stems from a work by George Russell. Miles Davis introduced the concept to the greater jazz world with "Kind of Blue" (1959), an exploration of the possibilities of modal jazz which would become the best selling jazz album of all time. In contrast to Davis' earlier work with hard bop and its complex chord progression and improvisation, "Kind of Blue" was composed as a series of modal sketches in which the musicians were given scales that defined the parameters of their improvisation and style.