Tom Service investigates the ostinato, a repeated phrase in music that can nag or hypnotise the listener (the word derives from the Italian for "stubborn "). From Ravel's Boléro to Donna Summer's "I Feel Love", the ostinato is everywhere in music - driving the crescendo of Rossini's William Tell overture, underpinning the primitive ritual dances of Stravinsky's Rite of Spring - but is it an accompaniment or a riff? A rhythm or a tune? Tom finds it can be all those things and more....