It was a well-intentioned idea designed to better match buyers and sellers, but it wound up complicating things. New exchanges quickly arose to attract customers, and what is blandly referred to as the stock market soon became a complex web of more than a dozen separate exchanges. Today, it’s not just the familiar New York Stock Exchange and Nasdaq, but also lesser-known ones with names like BATS and Direct Edge. Exchanges are now basically just technology companies, rooms full of computers that match buyers and sellers. There are also roughly 50 so-called dark pools, which allow traders to trade the same stocks that change hands on ordinary exchanges but don’t require participants to post as much information. (IEX will be a dark pool with plans to grow into a full-fledged exchange.) “I don’t think anybody who was putting these rules together envisioned we would have 13 exchanges,” Charles Jones, a finance professor at Columbia Business School, told me.

The recent proliferation of places to trade has forced exchanges to compete more aggressively for business. And because exchanges make money partly based on volume, it was natural that they would compete against one another to court high-speed traders. (High-speed trading has fallen off a bit from its peak a few years ago, but it still accounts for about half of all stock trades.) This competition, Katsuyama says, led the exchanges to put the needs of high-frequency traders before those of long-term investors. They created a new system of rebates and fees for different traders that, according to one analysis, cost long-term investors billions of dollars. Exchanges also started renting out space right next to the computers that powered the exchanges. Since information takes time to travel over networks, traders whose computers were next to the exchange’s computer would know what was happening on the exchange fractions of a second before traders whose computers were farther away. And those fractions of seconds could mean billions of dollars. No wonder Katsuyama felt cheated.

The beauty of the stock market is that it’s an astonishingly easy place for buyers and sellers to connect with one another. If you want to sell almost any stock, you can find a buyer within seconds and know within a few cents how much the buyer will pay. (Compare that with selling a house or a car or even an old piece of furniture on Craigslist.) In the old days, the stock market worked because there were people — so-called market makers — whose job was to ensure that there was almost always a willing buyer and seller for every stock. In the past decade, their jobs have been largely replaced by high-frequency traders who provide this middleman service. Over time, this shift to technology has generally made it cheaper for everyone, including long-term investors, to buy and sell stocks. But there are notable exceptions. A trader using a high-speed connection to jump in front of a deal between a willing buyer and seller is driving up costs for the buyer and isn’t really improving the market.

The goal with IEX, whose owners include mutual-fund companies and other big investors, is to attract only the high-frequency traders who add liquidity and keep away those looking for the kinds of advantages that Katsuyama says are unfair. IEX’s computers will be set up with a tiny delay designed to prevent the fastest traders from getting a jump on everyone else. IEX also won’t offer many of the special order types favored by high-frequency traders. Other exchanges have created complex structures of fees and rebates that have led to payouts for some traders and higher fees for others; at IEX, everyone who trades will pay the same amount. “People are always looking for ways to game the market,” Katsuyama says. “They’re always looking for ways to get an edge. It’s our job to make sure that edge doesn’t exist.”