Tom Service considers the semitone. Music's most fundamental building block, it can mean sorrow when it falls, triumph when it rises, but also provoke fear (in the theme from Jaws). It can become a glittering decoration when repeated as a trill. Tom talks to mezzo-soprano Sarah Connolly about the tragic falling semitones of Purcell's opera Dido and Aeneas, and also to musicologist Sarha Moore about the varied significances of the semitone in musical traditions of the Middle East and India, and its special effect in the riffs of Heavy Metal rock music.