find the best trading opportunity. In this sense, Nasdaq is largely a price quotation, not a trading system. While bid and asked prices can be obtained from the Nasdaq computer net- work, the actual trade still requires direct negotiation between the broker and the dealer in the security. However, in the wake of the stock market crash of 1987, Nasdaq instituted a Small Order Execution System (SOES), which is in effect a trading system. Under SOES, market mak- ers in a security who post bid or asked prices on the Nasdaq network may be contacted over the network by other traders and are required to trade at the prices they currently quote. Dealers must accept SOES orders at their posted prices up to some limit, which may be 1,000 shares but usually is smaller, depending on factors such as trading volume in the stock. Because the Nasdaq system does not require a specialist, OTC trades do not require a centralized trading floor as do exchange-listed stocks. Dealers can be located anywhere, as long as they can communicate effectively with other buyers and sellers. I. Introduction 3. How Securities Are Traded The McGraw−Hill Companies, 2001           CHAPTER 3 How Securities Are Traded 81     One disadvantage of the decentralized dealer market is that the investing public is vul- nerable to trading through, which refers to the practice of dealers to trade with the public at their quoted bid or asked prices even if other customers have offered to trade at better prices. For example, a dealer who posts a $20 bid and $20.30 asked price for a stock may continue to fill market buy orders at this asked price and market sell orders at this bid price, even if there are limit orders by public customers "inside the spread," for example, limit or- ders to buy at $20.10, or limit orders to sell at $20.20. This practice harms the investor whose limit order is not filled (is "traded through"), as well as the investor whose market buy or sell order is not filled at the best available price. Trading through on Nasdaq sometimes results from imperfect coordination among deal- ers. A limit order placed with one broker may not be seen by brokers for other traders be- cause computer systems are not linked and only the brokers own bid and asked prices are posted on the Nasdaq system. In contrast, trading through is strictly forbidden on the NYSE or Amex, where "price priority" requires that the specialist fill the best-priced order first. Moreover, because all traders in an exchange market must trade through the specialist, the exchange provides true price discovery, meaning that market prices reflect prices at which all participants at that moment are willing to trade. This is the advantage of a centralized auction market. In October 1994 the Justice Department announced an investigation of the Nasdaq stock market regarding possible collusion among market makers to maintain spreads at artifi- cially high levels. The probe was encouraged by the