The core original lineup of Soft Machine—Mike Ratledge, Robert Wyatt, and Kevin Ayers—made whimsical psych-pop with an anarchic creative bent. Bassist and composer Hugh Hopper (who replaced Ayers) moved the band into a more experimental, jazz-influenced direction that culminated with Third, a double album consisting of four side-long songs that incorporated extended improvisatory passages. When drummer, singer, and songwriter Wyatt left following '71's all-instrumental Fourth, the group moved into a strictly jazz-fusion sound for the rest of their career.