By the time the credit "Music by John Williams" appears on the screen, the simple repetition of two notes (F and F sharp) performed in a low register on a cello have already established the presence of the shark. When Williams first played the theme to Spielberg on the piano, the director - who himself is a fairly accomplished musician - thought he was kidding around. His first instinct was to dismiss the idea as too simple, but when he heard it again he knew that they were on to something.
The musical terminology for the "Jaws Theme" is ostinato, which comes from the Italian word meaning stubborn or obstinate. In rock and roll, it's known as a riff, the repetitive hook that makes a song like The Knack's My Sharona so irresistible. Essentially, Williams's two note theme is a musical expression of the single-mindedness of the shark, and its very lack of complexity means that it can be put through innumerable permutations. In the opening credits the two notes are first heard separately, spaced out by an ominous silence. They then come together and gather speed, just as a shark might move towards its victim. By varying the tempo, Williams was able to suggest both lurking menace and savage ferocity.
Two composers that Spielberg does not mention, however, are Igor Stravinsky and Claude Debussy. If you listen to The Rite of Spring and La Mer you can hear the influences. Williams was, in fact, following in the Hollywood tradition of mining the classics for thematic material, and he would go on to plunder Holst and Haydn for Star Wars, even lifting passages from Stravinsky for the Tatooine desert scenes. This is not in any way a criticism of his contribution to film scoring, which is more about selecting the appropriate music for a scene than composing it afresh. Williams's instincts for Jaws were spot on - you only have to play his theme against any shot of water to realise how effective his idea was.
