The Doobie Brothers had two distinct phases during their 1970s peak, evolving from boogie rockers with a penchant for mellow good vibes into a smooth blue-eyed soul outfit. Subsequent reunions and decades as a successful live attraction blurred the divide between the rambling "Black Water" and funky "What a Fool Believes," the band's two number one hits on Billboard. The Doobies racked up numerous other hits in both incarnations, songs that wound up as classic rock perennials.