Trading done before the regular market opens.

Investopedia Says:
Participation by market makers and ECNs is voluntary, so in pre-market trading there may be less liquidity than during normal hours of trading. If you are trading in pre-market or after-hours, you should always use a limit order.

Related Links:
Ever wonder what happens behind the scenes when you buy or sell a stock? Read on and find out! The Nitty-Gritty Of Executing A Trade
Find out the various ways in which a broker can fill an order, which can affect costs. Understanding Order Execution
Learn about the systems that run the market. Topics include market makers, specialists, SuperDOT, ECNs, SOES, Level I, II, and III Access, and more. Electronic Trading Tutorial

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