Hard Bop (1955-70) is an extension of Bebop that was somewhat interrupted by the Cool sounds of West Coast Jazz. The melodies tend to be more "soulful" than Bebop, borrowing at times from Rhythm & Blues and even Gospel themes. The rhythm section is sophisticated and more diverse than the Bop of the 1940's. Pianist Horace Silver is known for his Hard Bop innovations.
By the mid 1960's, Hard Bop had split into Post Bop, Modal Jazz and Soul Jazz. Hard Bop emerged as a major influence again in the early 1990's.