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Bebop! It's a rather silly word for a crucial chapter in jazz history. It didn't just come out of nowhere but evolved, fizzed and bubbled into existence in the USA in the early 1940s, as a result of a gloriously rich and complex musical chemistry involving different combinations of musicians, styles and places. All week, Donald Macleod and his special guest, the writer and broadcaster Geoffrey Smith, have some serious fun investigating this amazing musical phenomenon.

In Wednesday's programme, they visit the engine-room of jazz - the rhythm section - and in particular, Bebop's two key drummers, Kenny 'Klook-Mop' Clarke and Max Roach. Clarke's innovation was to shift the drummer's time-keeping function to the ride cymbal, leaving the snare and bass drum free to 'drop bombs' - unexpected offbeat accents - that perfectly complemented the way that the most innovative jazz musicians were beginning to play. In the event, Clarke was shipped off to Europe as part of the US contribution to the war effort, and he missed Bebop's explosion onto the scene in 1945. His shoes were filled by Max Roach, a percussion virtuoso who absorbed and extended Clarke's innovations. Donald Macleod and Geoffrey Smith explore the contributions of both men to a stellar sequence of recordings, with Charlie Christian, Charlie Parker, Dizzy Gillespie, Fats Navarro, Bud Powell, Thelonius Monk, Miles Davis, the Modern Jazz Quartet and Clifford Brown.