Market Microstructure: How Trades Actually Execute

The Modern Market Structure

U.S. equity markets are fragmented across more than a dozen registered exchanges (NYSE, Nasdaq, CBOE, and others) and numerous alternative trading systems (ATSs), including dark pools. When you place an order, your broker routes it to one of these venues — or to a market maker — based on a combination of regulatory requirements, payment arrangements, and execution quality metrics.

Market Makers

Market makers are firms that continuously quote both bid and ask prices for securities, standing ready to buy or sell at those prices. They profit from the bid-ask spread and manage their inventory risk through hedging. Market makers provide liquidity — without them, finding a counterparty for every trade would be far more difficult and time-consuming.

In exchange for providing liquidity, market makers receive payment for order flow (PFOF) from retail brokers — a controversial practice where brokers route customer orders to market makers who pay for the privilege. The SEC has scrutinized PFOF because it creates potential conflicts of interest between brokers and their customers.

Order Types and Execution

A market order executes immediately at the best available price. In a liquid market, this is typically very close to the quoted price. In an illiquid market or during volatile conditions, the execution price may differ significantly from the quoted price — this difference is called slippage.

A limit order specifies the maximum price you will pay (buy) or minimum price you will accept (sell). It will only execute at your specified price or better. Limit orders provide price certainty but not execution certainty — the order may not fill if the market never reaches your price.

Dark Pools

Dark pools are private trading venues where large orders can be executed without displaying the order to the public market. They are used primarily by institutional investors who want to trade large blocks without moving the market price. Dark pool trades are reported after execution but not displayed in real-time order books.

Dark pools account for a significant portion of U.S. equity trading volume. They are legal and regulated, but their opacity raises questions about price discovery and fairness.

Best Execution

Brokers have a regulatory obligation to seek best execution for customer orders — routing orders to venues that provide the most favorable terms considering price, speed, and likelihood of execution. What constitutes best execution is not always straightforward, and the SEC has ongoing initiatives to improve execution quality standards.

Educational Context

This article is for educational purposes only and does not constitute investment advice. Execution quality varies by broker, security, and market conditions.