Beyond that, it is a global expression of cultural modernity. Everyone in the world with media access knows what the Sydney Opera House looks like. Even Paris's celebrated Pompidou Centre, by Richard Rogers and Renzo Piano, is a local hero in comparison. First designed in 1956 and finally declared complete in 1973, the opera house was the single best known modern building in the world until the arrival of Frank Gehry's equally extraordinary Bilbao Guggenheim in 1997. But it will outlive the Guggenheim as an international architectural icon - because it did all the difficult work first.

In the pantheon of classic modern buildings, Utzon's creation has the status of myth. The myth states that the unknown Utzon, then in his 30s, submitted rough sketches - back-of-the-envelope stuff - to the competition judges, that he ignored most of the rules, that his design was only selected after being plucked at the last moment from the reject pile by one of the judges, architect Eero Saarinen, and that - the clincher, this - the design was unbuildable.

This last claim has some truth to it. Utzon's competition entry was a great deal less rough-and-ready than legend has it and was never actually rejected. But, in the late 1950s, nobody had ever tried to build anything quite so audacious as the overlapping shell roof structure, rising to 197 feet high, that Utzon envisaged - an abstract response, perhaps, to the waves and sailing boats of Sydney Harbour, but also intended as a way to conceal the usually lumpen protuberances of flytowers. In all the battles that followed - many of them with philistine politicians - Utzon may have lost control of his project, but won the battle to get his extraordinary roofscape built. It needed all the skills of the Anglo-Danish structural engineering company Ove Arup to find a way to make it work.