the 1700s the Amsterdam Stock Exchange had close links with the London Stock Exchange, and the two would often list each other's stocks. To clear the trades, time was required for the physical stock certificate or cash to move from Amsterdam to London and back. This led to a standard settlement period of 14 days, which was the time it usually took for a courier to make the journey between the two cities. Most exchanges copied the model, which was used for the next few hundred years. With the advent of the computer in the 1970s and 1980s, there was a move to reduce settlement times in most exchanges, leading by stages to a current standard of two days, known as T+2. With the advent of electronic settlement, and a move to dematerialisation of securities, standardised clearing systems were required, as well as standardised securities depositories, custodians and registrars. Until this point, many exchanges would act as their own clearing house, however the additional computer systems required to handle large volumes of trades, and the opening of new financial markets in the 1980s, such as the 1986 big bang in the UK, led to a number of exchanges separating or contracting the clearing and settlement functions to dedicated organisations. In some specialist financial markets, clearing had already been separate from trading. One example was the London Clearing House (later renamed LCH.Clearnet), which, since the 1950s, cleared derivatives and commodities for a number of London exchanges. Clearing houses who clear financial instruments, such as LCH, are generally called central counterparties (CCPs). In the wake of the financial crisis of 2007–08 the G20 leaders agreed at the 2009 Pittsburgh Summit that all standardised derivatives contracts should be traded on exchanges or electronic trading platforms and cleared through central counterparties (CCPs). Although some derivatives were already traded on exchange and cleared, many over-the-counter derivatives that met the criteria needed to be novated to CCPs as a result. United States clearing system The United States clearing system is the largest clearing system in the world. Millions of transactions, valued in the trillions of dollars, are conducted between sellers and purchasers of goods, services, or financial assets daily. Most of the payments making up the transactions flow between several banks, most of which maintain accounts with the Federal Reserve banks. The Federal Reserve therefore performs an intermediary role, clearing and settling international bank payments. Prior