Whenever a militant's bomb disrupts the flow of oil from Iraq, or the US government issues monthly unemployment statistics, a forest of hands and arms rises in the trading pits of the big US futures exchanges in Chicago and New York. In so-called “open outcry”, traders rush to protect themselves and their clients by buying futures contracts - helping to guard against, or speculate on, future movements in the price of oil or government bonds.

Away from the trading pits, however, many more billions of dollars of futures trades are being carried out electronically, by traders seated at computer screens with no need for a middle man. The opportunity to trade in this way, opened up by technological change, is doing more than simply highlight the gradual death in the US of the open outcry system. It heralds a deeper revolution for the US exchanges that deal in derivatives products such as futures and options, forcing them to tear up business models that have sustained them for more than a century.