Market microstructure fundamentals

#The order book

The order book (also called the limit order book, or the DOM — depth of market — in trading-platform language) is a live, price-ranked ledger of every resting order on an exchange for one instrument: bids (resting intent to buy) on one side, asks/offers (resting intent to sell) on the other, grouped by price level. The best bid and best offer together are the "top of book" or NBBO-equivalent; everything below that is "depth." [Established] — standard exchange terminology, consistent across CME Group, Nasdaq, and every major electronic venue's own documentation.

Depth matters because it's the mechanism behind the price-impact relationship in The mechanism: a given amount of aggressive order flow moves price less when there's more resting size stacked at nearby price levels to absorb it, and more when the book is thin. Reading depth, not just the last price, is the entire practical point of this course.

#The bid-ask spread

The spread — best offer minus best bid — is the cost of demanding immediacy. It compensates whoever is quoting it (a designated market maker, or simply another trader resting a limit order) for two risks: inventory risk (holding a position they didn't necessarily want, waiting for the other side to show up) and adverse selection risk (the resting order gets filled specifically because the price was about to move against it — see The mechanism). Spreads widen mechanically around news events and in thin, volatile conditions precisely because both risks rise together. [Established]

#How an exchange actually matches an order

This is where most retail explanations get vague, and where this course tries not to. CME Group's own Globex specification documents several distinct matching algorithms, configured per-product, and which one applies changes what "priority" in the book actually means:

• FIFO (first-in-first-out): price and time are the only criteria. At a given price level, whoever's resting order arrived first gets filled first. This is the algorithm most people mean when they say "price-time priority." [Established]
• Pro-rata: an incoming order is allocated across all resting orders at the best price in proportion to their size, not their arrival time. A large resting order gets a proportionally larger share of any incoming fill regardless of when it was placed. Several interest-rate futures products use this. [Established]
• Split FIFO/pro-rata and "TOP" priority: hybrid configurations exist — a fixed percentage of an incoming fill goes to FIFO allocation and the rest to pro-rata, or an order that improves the best price at time of arrival gets matched first regardless of size ("TOP" priority). [Established]

Source: CME Group's own Globex matching-algorithm documentation (CME Group Client Systems Wiki). This is worth knowing specifically because it changes what a resting order at the top of book actually signals — in a FIFO product, size sitting at the best price says less about intent than how long it's been there; in a pro-rata product, a single very large resting order can dominate fills at that price regardless of queue position, which changes how you read a large static level in the book.

#The maker-taker model, and why it exists

In the maker-taker fee model, the trader whose resting order gets filled (the "maker," providing liquidity) is paid a rebate; the trader whose aggressive order crossed the spread (the "taker," removing liquidity) pays a fee. The exchange is compensating market makers for quoting size, funded by charging the traders who consume that liquidity. [Established]

This isn't a minor accounting detail — it's regulated directly. SEC Regulation NMS Rule 610(c) caps the access fee an exchange can charge for a protected quote. That cap was 30 mils (30 cents per 100 shares) for years; the SEC's September 2024 amendment lowered it to 10 mils (10 cents per 100 shares, i.e. $0.001/share) for stocks priced at $1.00 or more, specifically to compress the size of maker-taker rebates and reduce their distortive effect on order routing. [Established] — SEC/Federal Register, Regulation NMS: Minimum Pricing Increments, Access Fees, and Transparency of Better Priced Orders, 2024 amendment. The SEC itself has stated the rebate structure can distort order routing without banning it outright — worth knowing because it means the maker-taker model is an actively contested piece of market structure, not settled physics, and futures markets (CME) largely don't use it the way equities markets do.

#Why this lesson matters before you read a single chart

Everything downstream in this course — order flow imbalance, absorption, iceberg detection — is an interpretation layered on top of these mechanics. Get the mechanics wrong (assume FIFO where the product is actually pro-rata; assume a resting bid means the same thing on every venue) and the interpretation built on top is reading noise. Order flow imbalance and absorption is the next lesson, and it assumes this one.