In short
- The order book = resting bids and asks with depth; a quote is the top of it, for limited size.
- Spread cost ≈ spread ÷ price; slippage = average fill − best quote — both paid inside the price.
- Market orders fix execution and risk price; limit orders fix price and risk execution.
- Zero commission ≠ zero cost — thin books and frequent trading turn invisible costs into a real drag.
After this lesson you can
- Read an order book's bids, asks and depth at each level
- Calculate spread cost and slippage for orders of different sizes
- Choose between market and limit orders based on the tradeoff each accepts
The order book, concretely
Path 1 introduced the spread as liquidity's price tag; the market-liquidity lesson showed depth vanishing in stress. This lesson opens the machine. An order book is two stacked lists of resting limit orders: bids (buyers waiting, highest first) and asks (sellers waiting, lowest first). The gap between the best bid and best ask is the spread; the quantities resting at each level are the depth. Every quote you see on a trading screen is the top of this book.
Key point: A quote is not 'the price' — it is the best currently-resting offer on each side, for a limited quantity.
A book in miniature
Suppose the book for a stock shows — asks: 200 shares at $50.10, 300 at $50.15, 500 at $50.25; bids: 250 at $50.00, 400 at $49.95, 600 at $49.85. The spread is 50.10 − 50.00 = 10 cents (0.2%). Buy 100 shares at market: you pay $50.10 — done at one level. Now buy 600 at market: 200 fill at $50.10, 300 at $50.15, and the last 100 at $50.25 — average $50.15, which is 5 cents worse per share than the quote you saw. That 5 cents is slippage: the cost of demanding more size than the best level holds. (All numbers illustrative.)
The two execution costs
Round-trip spread cost = Spread ÷ Price · Slippage = Avg fill − Best quote
In the book above: a small round trip costs 0.10 ÷ 50.05 ≈ 0.2% — buy at the ask, sell at the bid, and that gap is gone before anything else happens. The 600-share buy paid slippage of 50.15 − 50.10 = $0.05/share ($30 on the order) ON TOP of the spread. Scale matters: costs that look like noise per trade compound over many trades — 0.2% paid 50 times is a ≈9.5% drag (1 − 0.998^50), the compound lesson working against you.
Market vs limit orders — the tradeoff
| Market order | Limit order | |
|---|---|---|
| What you fix | Execution — you WILL be filled | Price — you will NOT pay worse than your limit |
| What you risk | Price: slippage, especially in thin or fast markets | Execution: the fill may never come; the market may run away |
| Best suited for | Deep, calm books; urgent exits | Thin books, volatile moments, patient entries |
| Hidden failure | 'Filled instantly' at a price you didn't examine | 'Saved' pennies on the limit, missed the move you researched |
Before pressing the button
A pre-order habit that costs ten seconds:
- Look at the spread in PERCENT of price — under ~0.1% is tight; whole percents demand respect.
- Compare your order size with the depth at the best level — bigger than it means slippage is coming.
- In calm, deep books a market order is usually fine; in thin or fast conditions, set a limit and accept fill risk consciously.
- After filling, compare your average fill to the quote you saw — that difference is the execution cost you actually paid.
Common trap: 'free trading' and invisible costs
Zero-commission does not mean zero-cost: the spread and slippage are paid inside the price, invisible on any statement. This matters most exactly where beginners experiment — small-cap stocks and thin crypto pairs, where a 1–2% round trip silently consumes weeks of expected returns per trade. High trade frequency multiplies the leak. The volume lesson's caveat completes the picture: on crypto, check the book on YOUR venue — the same coin can be tight on one exchange and treacherous on another.
Practise what you just read
Take this lesson's quiz and review cards in the app, then try the idea with virtual money in paper trading. Free account, no card.
Education only, not investment advice. Examples are illustrative. Read the full disclaimer