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On a professional trader's computer screen, there is a "ledger" or set of numbers separated by a fine line - buy offers and sell offers. A client wants to buy a million shares of GOOG on a "best execution" basis. The trader looks at the offers to sell GOOG which are all over the place: $582.34, $582.12, $582.23, etc. He is obligated to take the "best bid" or best price for his client. That is, the cheapest price. But sometimes the cheapest price is only good for 100 shares. In that case, the trader takes the smaller chunks in pieces starting from best bid until he can fulfill the million-share order.