Trading and Exchanges Chapter 6: How Order-Driven Markets Match Trades
Chapter 6 is where Harris explains the actual machinery that matches buyers with sellers. If you ever wondered what happens between the moment you hit “buy” and the moment your order fills, this is the chapter.
Order-Driven Markets Are Everywhere
Almost every major exchange in the world is an order-driven market. No dealer in the middle deciding prices. Just rules, orders, and a matching engine.
There are four flavors: oral auctions (pit trading you see in movies), single price auctions, continuous electronic auctions, and crossing networks. They look different on the surface, but they all rely on two things: order precedence rules (who gets matched first) and trade pricing rules (what price the trade happens at).
The Pecking Order: Who Goes First?
Every order-driven market ranks orders by precedence. The primary rule is always price priority. Buyers willing to pay the most go first. Sellers willing to accept the least go first. This is self-enforcing because everyone wants the best price.
What happens when two orders have the same price? That is where secondary precedence rules kick in. The most common: time precedence. Whoever submitted first gets filled first.
Here is a concrete example. Say orange juice futures are 103.10 bid, offered at 103.25. Guy is the bidder at 103.10 with time precedence. If you also want to buy at 103.10, tough luck, you wait behind Guy. Want to jump ahead? Bid 103.15. Now you have price priority over Guy and time precedence at the new price. If Guy wants the lead back, he bids 103.20. This leapfrog game is how prices tighten and liquidity improves.
Time precedence only matters when the tick size (minimum price increment) is meaningful. When the US switched from fractions to pennies in 2001, the tick went from 6.25 cents to 1 cent. That tiny tick basically killed time precedence because jumping ahead costs almost nothing. Displayed order sizes dropped.
Some exchanges also use display precedence (visible orders before hidden ones) and public order precedence (customer orders before exchange member orders).
The Order Book and How Matching Works
In a continuous electronic market, the exchange keeps an order book with all standing buy and sell orders sorted by precedence. When a new order arrives, the system checks: can this trade with anything on the other side?
If a new buy order bids at or above the best offer, it is marketable. The system matches it with the best-priced sell order. If the buy is bigger, the remainder matches with the next sell order. This continues until the order fills or no more trades are possible. If the new order is not marketable, it sits in the book and waits.
Harris walks through a step-by-step example. Imagine an empty book. Bea puts in a buy at 20.0. Sam puts in a sell at 20.1. They cannot trade, both sit in the book. Then Sol submits a sell at 19.8. That is below the best bid, so it immediately matches with Bea at 20.0 (the standing limit order price). Sol wanted at least 19.8, got 20.0. Nice deal for Sol.
Later, Bif submits a market buy for 4 units. It eats through Sam’s sell at 20.1 (2 units) then Stu’s sell at 20.2 (2 units). Bif pays an average of 20.15 across two trades.
Discriminatory vs Uniform Pricing
This is a key distinction. Discriminatory pricing means each trade happens at the limit price of the standing order. If your big buy order matches three sell orders at different prices, you pay three different prices. First chunks are cheap, last chunks are expensive. This is what continuous markets use.
Uniform pricing means everyone trades at one market-clearing price. The exchange collects all orders, finds the price where supply equals demand, and all trades happen there. This is what single price auctions use.
Who benefits from which? Large aggressive traders prefer discriminatory pricing because they get better prices on early fills. Small limit order traders prefer uniform pricing because they do not get picked off.
But here is the catch: in continuous markets, you cannot enforce uniform pricing. A big trader can split their order into small pieces and submit them one at a time, getting discriminatory pricing anyway. To use uniform pricing, the market has to stop and run a call auction.
Harris shows that for the same set of orders, the single price auction produces higher total trader surplus (1.6 vs 1.0 in his example). But the continuous auction lets you trade whenever you want. That immediacy has value too.
Crossing Networks: The Quiet Rooms
Crossing networks do not discover prices at all. They take prices from other markets and match buyers with sellers at those imported prices. POSIT, for example, crosses stocks eight times a day at the midpoint of the bid-ask spread.
Upside: zero market impact, low commissions, anonymity. Downside: less than 10% of submitted orders actually fill. And there are two dangers. First, stale prices. If a stock’s value changes after the crossing price was set, informed traders will exploit that gap. Second, price manipulation. If you know you are buying 500,000 shares at whatever the primary market shows at 1:30 PM, you might submit a small sell order at 1:29 to push the price down. Costs pennies, saves thousands.
Why This Chapter Matters
The rules of the game determine who wins. Price priority, time precedence, tick size, discriminatory vs uniform pricing. These directly affect whether your order fills, at what price, and who profits at your expense. Trade without understanding this and you are bringing a spoon to a knife fight.
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