Order routing and best execution: NBBO, Rule 611, PFOF and 606

When you tap "buy" in a brokerage app, your order doesn't go straight to the New York Stock Exchange. It goes to your broker, which decides where to send it: an exchange, a wholesale market maker, or an alternative trading system. That decision, order routing, happens in milliseconds, and it determines the price you get to a fraction of a cent. A set of US rules governs it: a consolidated national best price that everyone can see, a rule against trading through better displayed prices, a duty on brokers to seek the best execution reasonably available, and public reports that let you check how they did.

This page traces an order's path and explains each of those rules, with dates and citations as of October 2026. It also explains payment for order flow, the practice of wholesalers paying brokers for retail orders, which is legal in the US and the reason most retail market orders never reach an exchange.

The path of an order

1. You submit an order in your broker's app.
2. Your broker checks it (buying power, restrictions) and decides where to send it. The broker can also fill some orders itself.
3. The order goes to a venue:
   • a wholesaler (a large market maker that trades against retail orders from many brokers);
   • an exchange, where it either trades against displayed orders or rests on the order book;
   • an alternative trading system (ATS), including "dark pools" that don't display their orders.
4. The execution is reported to the consolidated tape, and a confirmation returns to you.

Which venue a broker chooses depends on its routing arrangements, the order's type and size, and market conditions. Stock exchanges explained: NYSE, Nasdaq and where your order goes describes the venues themselves; this page focuses on the rules for choosing among them.

The NBBO: one best price for a fragmented market

US stocks trade on more than a dozen exchanges at once. Each exchange has its own best bid and offer. The securities information processors (SIPs) collect every exchange's top-of-book quote and publish the consolidated result: the national best bid and offer (NBBO). If Exchange A bids $25.00 and Exchange B bids $24.99, the national best bid is $25.00. If Exchange C offers at $25.02 and Exchange A at $25.03, the national best offer is $25.02.

The NBBO is the benchmark almost everything else is measured against: whether a trade "traded through" a better price, whether a fill got "price improvement," and how wide a stock's spread is. Many brokerage apps show the NBBO as the bid and ask. The Bid-ask spread: what it is, what it costs and how to read it page covers what the spread costs.

Rule 611: the order protection rule

Regulation NMS, adopted by the SEC in 2005, includes Rule 611, the order protection or "trade-through" rule. It requires trading centers to establish, maintain and enforce written policies reasonably designed to prevent trade-throughs: executions at a price worse than a protected quotation displayed on another venue. A protected quotation is an automated, immediately accessible best bid or offer displayed by an exchange.

The rule has exceptions. The most important is the intermarket sweep order (ISO): a sender can mark an order as an ISO if it has simultaneously sent orders to take out every better protected quote. Rule 611 protects only displayed top-of-book quotes, not hidden orders or depth behind the best price.

FINRA Rule 5310: best execution

Rule 611 is about venues. Best execution is about your broker. FINRA Rule 5310 requires a broker-dealer handling a customer order to use reasonable diligence to find the best market for the security and to buy or sell there so that the result for the customer is as favorable as possible under prevailing market conditions.

The rule lists factors a broker must weigh, including:

• the character of the market: price, volatility, relative liquidity and pressure on communications;
• the size and type of the transaction;
• the number of markets checked;
• the accessibility of the quotation;
• the terms and conditions of the customer's order.

Best execution is not "the best price that ever existed." It's a standard of diligence. Brokers that route orders in bulk rather than reviewing each one must conduct "regular and rigorous" reviews of execution quality, at least quarterly, broken down by security and order type, and compare the quality they get against other venues they could use (Supplementary Material .09 to Rule 5310). A broker that receives payment for order flow keeps the full best-execution duty.

Wholesalers and payment for order flow

Most marketable orders from US retail brokers go to a small number of wholesalers. A wholesaler fills the order from its own inventory, usually at the NBBO or slightly better, and manages the risk across millions of orders. Retail orders are attractive to wholesalers because, on average, they are small and unlikely to come from traders who know where the price is about to go.

Payment for order flow (PFOF) is what a wholesaler pays the broker for sending those orders. The SEC's Investor.gov describes the amount as "perhaps a penny or more per share." PFOF is one reason many brokers can charge zero commission on stock trades. It is also a conflict of interest: a broker might be tempted to route to the venue that pays most rather than the one that executes best. Disclosure and the best-execution duty are the regulatory answers.

Whether PFOF helps or hurts retail investors overall is debated. The practical point for an individual is narrower: compare a broker's execution-quality statistics, not just its commission.

Rule 606 and Rule 605: checking the work

Two SEC disclosure rules make routing visible.

Rule 606 (order routing). Brokers publish quarterly reports, with monthly breakdowns, on where they route customers' non-directed orders in NMS stocks and listed options. The reports name the top venues, show the share of orders by type (market, marketable limit, non-marketable limit, other), and disclose the net payment for order flow received per venue, along with a description of the material terms of those arrangements. On request, a broker must also tell an individual customer where their own orders were routed over the previous six months.

Rule 605 (execution quality). Market centers publish monthly statistics on how orders executed: speed, effective spreads, and how often orders received price improvement. Amendments adopted on March 6, 2024 widened the rule to cover larger brokers and added new measures, including a summary report alongside the detailed one. Under an extension published in October 2025, the compliance date moved from December 14, 2025 to August 1, 2026, so the first reports, covering August 2026, were due by the end of September 2026.

What's changing, and what isn't

In 2024 the SEC adopted two related changes to Regulation NMS: a half-penny ($0.005) quoting increment for certain tightly quoted stocks (Rule 612) and lower caps on the fees exchanges can charge to access their quotes, from $0.003 to $0.001 per share (Rule 610). As of October 2026, neither is in effect. The compliance date was postponed twice, most recently on June 11, 2026, to the first business day of November 2027, and the SEC chair directed staff to review both rules, including whether the increments and caps should change. Penny quoting and the $0.003 access-fee cap still apply.

How to evaluate your broker's routing

Pitfalls and misconceptions

• "Commission-free means free." The cost moved into the spread you pay and the price improvement you do or don't get.
• "My order went to the NYSE because the stock is listed there." Listing and execution venue are separate. Most retail orders execute off-exchange.
• "Best execution means the best price." It's a diligence standard that weighs price, speed, likelihood of execution and other factors.
• "Rule 611 protects every price I can see." It protects displayed top-of-book exchange quotes, not depth or hidden liquidity.

How Finabase helps

Finabase doesn't route orders, so it has no routing arrangements or payment for order flow. Its quote pages show the live bid and ask with their sizes, and the last trade, which lets you compare the fill on your broker's confirmation with the market at the time of the trade.

Frequently asked questions

What is the NBBO?

The national best bid and offer: the highest bid and the lowest offer for a stock across all US exchanges at a given moment, published by the securities information processors. It's the benchmark for whether a trade got a fair price.

Is payment for order flow legal in the US?

Yes. As of October 2026, US brokers may accept payment for order flow for stock and options orders, provided they disclose it, including in their Rule 606 reports, and continue to meet their best-execution obligations under FINRA Rule 5310.

What does best execution mean?

It means a broker must use reasonable diligence to obtain the most favorable terms reasonably available for a customer's order under prevailing market conditions. Price matters most, but speed, likelihood of execution and order size are also considered.

How can I see where my orders are routed?

Read your broker's quarterly Rule 606 report, usually linked from its website's disclosures page. You can also ask the broker for a report on where your own orders were routed over the previous six months.

Summary

A US stock order passes through your broker's routing decision to a wholesaler, exchange or ATS. The NBBO defines the best displayed prices; Rule 611 stops venues from trading through protected quotes; FINRA Rule 5310 requires brokers to seek the most favorable terms reasonably available and to review execution quality regularly. Payment for order flow funds commission-free trading and creates a conflict that disclosure is meant to manage, and Rule 606 and 605 reports are where you can check how well your orders are handled.