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In the stock market, why is the “open” price value never the same as previous day's “close”?
It's done by Opening Auction (http://www.advfn.com/Help/the-opening-auction-68.html): The Opening Auction Between 07.50 and   a random time between 08.00 and   08.00.30, there will be called an auction period during which time,   limit and market orders are entered   and deleted on the order book. No   order execution takes place during   this period so it is possible that the   order book will become crossed. This   means that some buy and sell orders   may be at the same price and some buy   orders may be at higher prices than   some sell orders. At the end of the random start period,   the order book is frozen temporarily   and an order matching algorithm is   run. This calculates the price at   which the maximum volume of shares in   each security can be traded. All   orders that can be executed at this   price will be filled automatically,   subject to price and priorities. No   additional orders can be added or   deleted until the auction matching   process has been completed. The opening price for each stock will   be either a 'UT' price or, in the   event that there are no transactions   resulting form the auction, then the   first 'AT' trade will be used.