After You Hit Buy: How Your Trade Finds a Market

One stock, many markets

When you decide to buy or sell a stock, do you ever think about where the order goes? Most of us don't. We hit the button, wait a moment, and assume it's handled. In reality, your order enters a quiet competition among marketplaces, and a piece of software acting on your broker's behalf decides which one should see it first.

That wasn't always the case. Not too long ago, U.S. stock trading was a simpler map. In 2005, about eight exchanges traded these stocks. At the time of the SEC's June 2026 proposal, there were 17 operating exchanges, with more approved. Add in other types of trading venues and you can see why market people talk about "fragmentation": the pool of buyers and sellers for any stock is spread across many sites. That's why routing matters. A route is the path an order takes to a venue, and the goal is a "fill"—market slang for an order that actually meets a buyer or seller and becomes a trade.

A router is your order's navigator

When you don't specify a particular exchange, your order is called a non-directed order. You are, in effect, leaving the destination to your broker. This is where smart order routing comes in. The router is not a robot with a briefcase; it is software that monitors live prices and decides where to send your order next. Think of it as a navigation app for financial markets, but with a crucial difference: the route cannot simply be the fastest or cheapest. It has to respect a set of market-structure rules designed to keep price promises honest.

Modern markets move at microsecond speeds. The system's anchor is the national best bid and offer, or NBBO—the best displayed price to buy and the best displayed price to sell across the market. Rule 611 of Regulation NMS, often called the Order Protection Rule or the trade-through rule, sets the basic principle: a trade should not happen at a worse price when a better-priced protected quote is visible elsewhere.

The trade-through rule explains why you don't get skipped

Here is a concrete example. Suppose Exchange A is displaying an offer to sell a stock at $29.99. Exchange B should not simply execute a sell order at $30.00 as if the better price did not exist. Generally, it must first route to satisfy the $29.99 quote on Exchange A. That is what people mean by not "trading through" a protected quote.

This protection may sound abstract, but it matters for real people who post limit orders—orders that say "I'm willing to buy at this price, or sell at this price, but not worse." If a displayed quote could be ignored whenever convenient, fewer investors would dare show their hand by posting a price. The rule creates a basic trust: if you put a better price out there, the market has to notice it before executing at a worse one.

The catch is that this protection has a cost. Because every protected quote demands attention, brokers and trading venues find themselves connected to nearly every market, whether or not they expect much business there. In the SEC's proposal, the agency estimated that a broker-dealer connecting to all exchanges spends roughly $5.7 million a year on market data and connectivity, and that bringing a new exchange online can cost about $1.5 million, with ongoing maintenance on top of that. Large investors can face a different chore: before executing a large order, the system may have to stop at a small displayed order on another market, which could signal what they are doing and affect the final price.

Locked and crossed markets: when quotes don't make sense

Related rules keep the displayed market readable. A "locked" market happens when the bid on one venue equals the offer on another—for instance, both at $10. A "crossed" market is even stranger: one market's bid is actually higher than another's offer, such as a bid at $10 and an offer at $9.99. These contradictions can confuse anyone reading quotes, so exchanges are generally expected to avoid displaying them.

Today's regulators are asking whether such rules are still necessary. In June 2026, the SEC proposed to rescind the Order Protection Rule and the locked-and-crossed-markets prohibition. If adopted, this would be one of the biggest changes to U.S. equity market structure in two decades. Supporters say modern automated routing and real-time data have made the mechanical rules less necessary, and removing them could cut costs and complexity. Others worry that investors could lose a practical floor beneath the prices they see. The national best bid and offer would remain as a reference, but without the order protection rule, a visible best quote might no longer be a quote that is generally accessible.

Reading the road signs: public routing reports

Public disclosures do some of the transparency work. Under SEC Rule 606(a), brokers that route customer orders in national market system stocks and listed options must publish quarterly reports. The information is broken down by calendar month and posted on a free, public website for three years. For non-directed orders, the report shows the ten venues receiving the most orders and any venue receiving at least five percent. It also shows what share of each order type went where—market orders, limit orders, marketable limit orders and other orders—along with the net payments for order flow received, transaction fees paid and rebates received. In other words, you can see where your kind of order tends to travel and whether your broker has a financial relationship with the destination.

There is an important limit. A routing report is not the same as an execution-quality report. It tells you the route, not whether every execution was the best possible outcome. A separate rule, Rule 605, covers monthly execution statistics from market centers, with information meant to shed light on execution price and speed. Even those disclosures cannot capture every factor that matters for a particular order.

What you can ask your broker

If you want to know what happened to your own orders, the rules also give you some rights. Brokers have to tell you, on request, which venue received your order in the previous six months, whether the order was directed or non-directed, and when any resulting trades happened. They also have to notify customers in writing at least once a year that this information is available. For some larger institutional-style orders sent on a "not held" basis—meaning the broker uses judgment about timing rather than firing the order immediately—there are deeper reports with fill rates and fees, though those reports have their own eligibility thresholds. The lesson for the rest of us is simple: routing is not a secret.

The conversation is still open

None of this is settled. The proposal from June 2026 was open for public comment, and industry groups reminded regulators that Rule 611 does not operate alone. Change it and you also affect best execution, the NBBO, market data revenue and the incentives for venues to display liquidity. Even some supporters of removing the trade-through rule admit that not all complexity will disappear; brokers would still owe clients a duty of best execution and would still connect to many venues to find good prices.

What should an ordinary investor take from this? You don't need to watch every microsecond of routing decisions, but knowing that a decision exists is valuable. Your order has a destination, a navigator and a set of rules. If a better price is displayed in another market, the system is supposed to notice before it lets your order fill at a worse one. That principle is simple, even if the plumbing is not.