People trading stock will prefer to trade on the most popular exchange since this gives the largest number of potential counter parties (buyers for a seller, sellers for a buyer) and probably the best price. However, there have always been alternatives such as brokers trying to bring parties together to trade outside the exchange. Some third markets that were popular are Instinet, and later Island and Archipelago (the latter two have since been acquired by Nasdaq and NYSE, respectively). One advantage is that this avoids the commissions of the exchange. However, it also has problems such as adverse selection. Financial regulators are probing dark pools.
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Take for example the S&P 500 market index, which is comprised of 505 companies. Buying shares in 505 different companies would be very difficult to do. Thanks to mutual funds and ETFs, we can simply buy one single security that holds shares in all 505 companies. The largest S&P 500 mutual fund is the Vanguard 500 Index Fund Admiral Shares (VFIAX) and the largest S&P 500 ETF is the State Street Global Advisors SPDR S&P 500 ETF (SPY).