The price is not given.
It is quoted, by someone.
Most of finance starts with a price and works forwards. Microstructure starts one level down: a limit order book, a queue of resting orders, and a market maker deciding what to show. Order Flow teaches that machine from the inside — how orders match and where queue position gets its value, why the spread is compensation for adverse selection and inventory rather than a fee, how informed flow moves the price through Kyle's lambda, and how to execute a large order without paying away the alpha that justified it. This is the layer every trading interview probes and almost no textbook course covers.
A market is not a price — it is a book of resting orders and a rule for matching them. Meet the limit order book, the two order types everything else is built from, price-time priority and the queue it creates, and the life cycle of an order from submission to fill or cancel. Get this layer right and every later idea has somewhere to stand.
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Level 1 · ▶ NEXT A Market Is a Book ★★★ ◆◆
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Level 2 Limit Orders & Market Orders ★★★ ◆◆ 🧩 Builds on L01
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Level 3 Depth, the Touch & the Spread ★★★ ◆◆ 🧩 Builds on L02
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Level 4 Price–Time Priority & Queue Position ★★★ ◆◆◆ 🧩 Builds on L03
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Level 5 The Life of an Order ★★★ ◆◆ 🧩 Builds on L04
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👑Level 6 · OPEN The Book Open ★★★ ◆◆◆ 🧩 Builds on L05
Why does a spread exist at all? Not as a fee — as compensation. Follow a market maker's P&L to find the two risks being paid for: adverse selection, the chance the person hitting you knows something, and inventory, the risk of the position you did not want. Then decompose a real spread into those parts and price the information in a trade with Kyle's lambda.
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Level 7 The Market Maker's P&L ★★★ ◆◆◆ 🧩 Builds on L06
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Level 8 Adverse Selection ★★★ ◆◆◆◆ 🧩 Builds on L07
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Level 9 The Glosten–Milgrom Spread ★★★ ◆◆◆◆ 🧩 Builds on L08
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Level 10 Inventory Risk ★★★ ◆◆◆ 🧩 Builds on L09
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Level 11 Decomposing the Spread ★★★ ◆◆◆◆ 🧩 Builds on L10
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👑Level 12 · OPEN The Spread Open ★★★ ◆◆◆◆ 🧩 Builds on L11
Order flow carries information before the price does. Learn to measure imbalance at the touch, to recognise toxic flow that is picking you off systematically, to classify trades as buys or sells when nobody tells you, to find the liquidity that is deliberately hidden, and to understand why speed is worth paying for — a fast quote is an option to cancel.
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Level 13 Kyle's Lambda & Price Impact ★★★ ◆◆◆◆ 🧩 Builds on L12
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Level 14 Order Flow Imbalance ★★★ ◆◆◆ 🧩 Builds on L13
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Level 15 Toxic Flow & VPIN ★★★ ◆◆◆◆ 🧩 Builds on L14
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Level 16 Classifying Trades ★★★ ◆◆◆ 🧩 Builds on L15
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Level 17 Hidden Liquidity ★★★ ◆◆◆ 🧩 Builds on L16
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👑Level 18 · OPEN The Flow Open ★★★ ◆◆◆◆ 🧩 Builds on L17
Now sit on the other side: you have a large order and the market will move against you as you work it. Measure the true cost with implementation shortfall, model impact with the square-root law, meet the standard schedules and what they optimise, solve the Almgren–Chriss trade-off between impact and risk, and navigate the fragmented venue landscape you actually trade in.
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Level 19 Latency & the Queue Race ★★★ ◆◆◆◆ 🧩 Builds on L18
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Level 20 Implementation Shortfall ★★★ ◆◆◆ 🧩 Builds on L19
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Level 21 The Square-Root Law ★★★ ◆◆◆◆ 🧩 Builds on L20
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Level 22 VWAP, TWAP & Participation ★★★ ◆◆◆ 🧩 Builds on L21
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Level 23 Almgren–Chriss: Impact vs Risk ★★★ ◆◆◆◆◆ 🧩 Builds on L22
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👑Level 24 · OPEN The Final Open ★★★ ◆◆◆◆◆ 🧩 Builds on L23
Take the Book
Clear all 24 levels and the four opens and you'll be able to do the thing a trading floor actually screens for: look at a market and say who is on the other side of it. You'll read a book's depth and infer where the queue is worth standing, decompose a quoted spread into adverse selection and inventory, tell informed flow from noise, price the cost of crossing versus the risk of waiting, and schedule a large order so the impact you pay is the impact you chose. Then you'll sit behind the quotes yourself, in a live session where every fill you get is a fill somebody wanted to give you.
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