NEW YORK—Information about stock-market trades that used to flow through the floor of the New York Stock Exchange will now be kept secret from traders, thanks to a procedural change quietly implemented last week.

In a move rankling the floor's denizens, names of buyers and sellers on the storied exchange's "book" of orders will no longer be visible to traders.

Stock-exchange officials declined to explain why the change was made. European Pressphoto Agency

Originally recorded in a physical book, the exchange's orders are now transmitted electronically.

Designated market makers, who have obligations to keep trading smooth in specific stocks on NYSE Euronext's New York Stock Exchange, are no longer be able to see who has entered interest in buying and selling a stock before the trade is executed, according to a memorandum sent Jan. 18 to members of the stock exchange. The new guideline went into effect Jan. 19.

Traders learned of the change at an urgent 8 a.m. meeting held on Jan. 18, said someone familiar with the situation.

Floor brokers, accustomed to being able to share information about the interest in a stock with their clients, said the change will curtail their ability to facilitate trades directly between buyers and sellers. The floor brokers were often able to get identifying information about trading interest from the designated market makers.

"We used to be able to see who was in the crowd. Now they've taken that away from us," said Alan Valdes, director of floor trading at Kabrik Trading.

However, he said preserving the anonymity of all buyers and sellers helps keep trading as fair as possible. "It's the right move," he said.

Stock-exchange officials declined to explain why the change was made.

"We continuously review practices and procedures and, where necessary, modify them in the interests of our customers and all market participants," said spokesman Ray Pellecchia.

Other traders said knowing the identity of buyers and sellers helps them prevent erroneous trades from being executed. If a suspicious order is entered, designated market makers can sometimes verify whether the investor intended to place the order.

Still, many acknowledged that the identifying information has become less important in a lightning-fast market where an increasing amount of market activity occurs away from the operator of the Big Board.

Given the speed at which markets operate today, floor-based market makers are unlikely to be able to use identifying information to gain any advantage in making trading decisions, said Joseph Cangemi, chairman of the Security Traders Association. Traders "used to that information have to find a way to grow past that particular procedure of information gathering," he said.

Keith Bliss, director of sales and marketing at Cuttone & Co., said his firm wasn't upset by the rule and didn't expect to feel an effect from it.

"Bear in mind that a lot of our order flow is electronically received and delivered out to the marketplace right now," he said. "We're not really concerned about where the flows are actually coming from per se. Of course we want to know if there are large things going on that are going to impact the stock, but who's actually doing it is not of great interest to us in most cases."

Traders who fail to comply with the new guideline face disciplinary action, according to the memorandum.

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