Market Microstructure: How Orders Really Get Filled

Chapter 5 covered market-making at a conceptual level: bid, ask, spread. This deep dive goes one level deeper into the actual plumbing of modern equity markets — where orders really go, and why "the market" isn't the single, unified order book most people picture.

Markets are fragmented, not singular

There isn't one single "stock market" that all orders flow into. In the US alone, a stock can trade across more than a dozen different exchanges, plus a substantial volume that never touches a public exchange at all. Understanding where an order actually gets executed is a real, non-trivial part of market structure.

Lit markets vs. dark pools

A lit exchange (NYSE, Nasdaq) displays its order book publicly — anyone can see the current bids and asks and their sizes before a trade happens. A dark pool is a private trading venue that does not display orders publicly before execution — institutional investors use dark pools specifically to trade large blocks of stock without signaling their intent to the broader market first, since a visible large order can itself move the price against them before they finish executing (a real, well-documented effect called market impact).

Payment for order flow (PFOF)

Many retail brokers route customer orders not directly to a public exchange, but to a wholesale market maker, who fills the order (often at a price at least as good as the public best bid/ask, sometimes slightly better — "price improvement") and pays the broker a small fee for that order flow. This is why many retail brokers can offer commission-free trading — the economics shifted from charging the customer directly to being paid by the market maker instead. It's a genuinely debated structure: proponents point to real, measurable price improvement for retail customers; critics argue it creates a conflict of interest, since the broker's revenue depends on routing volume to whoever pays for it, not necessarily to wherever the objectively best execution is.

Smart order routers — an algorithm making the routing decision

A smart order router (SOR) is the system (used by brokers and by trading desks executing large institutional orders) that decides, order by order and often piece by piece, where to actually send a trade across all these fragmented venues — some to a lit exchange, some to a dark pool, sometimes broken into smaller pieces over time — to minimize cost and market impact. This is a real, substantial piece of technology and quantitative work inside a modern trading desk, distinct from the "spread capture" market-making role from Chapter 5, though the two interact constantly.

Why this matters for understanding a real fill

The price you actually get on a trade is the product of a whole invisible routing and execution decision, not just "whatever the market price was." A large institutional order executed carelessly can move the market against itself; the same order executed well, split across venues and over time by a good SOR, can achieve a meaningfully better average price. This is precisely why "execution" is its own real specialty within S&T, separate from pure market-making — getting a large order done cheaply is a genuine skill, not just a mechanical formality.