When people today hear the word jazz, the music that most often springs to mind is the small combo-based style known as Bop. Built on a rhythmic foundation of walking bass and a steady cymbal pulse, Bop music is geared toward virtuosic and often extended soloing over a series of predetermined chord changes. In the early 1940s, Charlie Parker and Dizzy Gillespie revolutionized jazz with their stripped-down aesthetic, speeded-up tempos, and radical harmonic innovations, a style that critics dubbed "Bebop" after a classic tune of the same name. By the mid-'50s, the music had evolved into the tougher sounds of Hard Bop outfits such as Art Blakey & the Jazz Messengers, who incorporated elements of Gospel and blues into slightly slower tempos. Post Bop groups, such as the Bill Evans Trio and mid-'60s Miles Davis Quintet, reshaped the basic elements of Bebop and Hard Bop, at times incorporating ideas associated with Avant-Garde Jazz.