The order book and the spread · 6 min
Every listed share trades on an order book: a queue of bids (buyers, highest first) and asks (sellers, lowest first). The best bid and best ask are the only two prices that matter at the moment you trade; the gap between them is the spread, and it is the first cost you pay.
A market order crosses the spread immediately — you take whatever the other side is offering. A limit order joins the book at your price and only fills if the market comes to you. Market orders buy certainty of execution; limit orders buy certainty of price. You cannot have both.
Liquidity is depth, not price. A large-cap index constituent may show thousands of shares at the touch; a small-cap may show a hundred. Trading size in a thin book moves the price against you before your order is done — this is slippage, and it is invisible on a chart.
Order types you should know · 5 min
Market: fill now at the prevailing price. Use when getting out matters more than the last cent.
Limit: fill at your price or better, or not at all. Use for entries where a worse price invalidates the idea.
Stop (stop-loss): a resting instruction that becomes a market order when a trigger price trades. It caps intent, not outcome — in a gap the fill can be far below the trigger.
Stop-limit: becomes a limit order at the trigger, so it protects your price but can leave you holding a falling position unfilled.
Time in force decides how long the order lives: day orders expire at the close, GTC orders rest until filled or cancelled.
Module quiz
You place a market buy in a thinly traded stock. What is the main risk?
A stop-loss guarantees you exit at the trigger price.