When you click "buy" or "sell" on a trading app, you never see the hidden machinery that matches your order. Behind the screen lies a living, breathing structure called a limit order book — a real-time record of who wants to trade, at what price, and how badly. This chapter pulls back the curtain. We'll see how the book stores resting orders, how aggressive trades sweep through it, and why prices end up where they do. Along the way, you'll discover why the spread between the best buy and sell prices exists, and how it protects market participants from being picked off by better-informed traders.