How do stock exchanges actually match buy and sell orders? I want to understand market microstructure.

Market microstructure is the study of how trading actually happens — the mechanics behind every stock transaction. There are two primary market structures:

Order-Driven Markets (e.g., NYSE Arca, most electronic exchanges):

Buyers and sellers submit orders to a central limit order book (CLOB). The exchange matches orders based on price-time priority:

1. Best price gets matched first
2. Among orders at the same price, the earliest order gets priority

Quote-Driven Markets (Dealer markets, e.g., most bond markets, NASDAQ historically):

Dealers (market makers) continuously post bid and ask prices. They stand ready to buy at the bid and sell at the ask, providing liquidity. Their profit is the bid-ask spread.

Key players:

• Market makers: Provide liquidity by quoting two-sided markets. They profit from the spread but bear inventory risk.
• Brokers: Act as agents, routing client orders to the best venue. They earn commissions.
• Designated market makers (DMMs): Specialists on the NYSE floor with obligations to maintain fair and orderly markets.

The bid-ask spread matters because:

• It represents the cost of immediacy — the price you pay for instant execution
• Narrower spreads = more liquid markets = lower trading costs
• Spreads widen during volatility, after-hours trading, and for less liquid stocks

Example:

Velox Technologies has a bid of $48.50 and an ask of $48.75. The spread is $0.25 (or 0.51%). If Elena buys 200 shares, she pays $48.75 per share. If she immediately sells, she receives $48.50 — losing $0.25/share to the spread.

Exam tip: Know the difference between order-driven and quote-driven markets, and understand that most modern exchanges are hybrid, combining elements of both.

Learn more about equity markets in our CFA Level I Equity course.