The mechanics of buying and selling — from basic market orders to advanced conditional orders. Master these tools and you control precisely how your trades get filled.
After this sub-module you will be able to:
1. Explain the bid-ask spread and how it affects your trading costs.
2. Use market, limit, stop, and stop-limit orders correctly.
3. Understand trailing stops and how they lock in profits.
4. Read a Level 2 order book and interpret depth of market.
5. Recognize when each order type is — and isn't — appropriate.
Before we discuss individual order types, you need to understand the structure they operate within. The order book (also called Level 2 data or depth of market) is a real-time ledger showing every outstanding buy and sell order for a security, organized by price.
The left side shows bids — orders from buyers willing to purchase at a given price. The right side shows asks (or offers) — orders from sellers willing to sell at a given price. The gap between the highest bid and the lowest ask is the spread.
This $0.02 spread is typical for a highly liquid stock like AAPL. Less liquid stocks may have spreads of $0.05–$0.50 or more. The spread is an invisible cost you pay on every trade.
When you buy with a market order, you're matched against the best available ask. When you sell with a market order, you're matched against the best available bid. The spread is effectively a transaction cost — you buy at a slightly higher price and sell at a slightly lower price than the "midpoint" of the market.
For a $190 stock with a $0.02 spread, this cost is negligible — about 0.01% per trade. But for a $3 penny stock with a $0.10 spread, you're paying over 3% just to enter and exit. Spreads matter enormously for active traders and for anyone trading less liquid instruments.
A market order tells your broker: "Buy (or sell) this security immediately at the best available price." It guarantees execution but not price. You will be filled against the best available orders in the book.
A limit order tells your broker: "Buy at this price or better" (or "Sell at this price or better"). It guarantees price but not execution. Your order sits in the book until someone is willing to trade at your price — or you cancel it.
Limit orders should be your default. Market orders are for emergencies — when you absolutely must get in or out right now regardless of price. For routine entries and exits, always use limit orders. The discipline of choosing a price forces you to think about what you're willing to pay, which is the foundation of good trading.
A stop order becomes a market order when the price reaches your specified trigger price. It's primarily used to limit losses — hence the common name "stop-loss." You set a price below your entry (for a long position) that says: "If the stock falls to this level, get me out."
A stop-limit order combines a stop trigger with a limit price. When the stop price is reached, instead of becoming a market order, it becomes a limit order at your specified limit price. This gives you price protection but introduces the risk that your order won't fill at all.
A trailing stop automatically adjusts your stop price as the stock moves in your favor. You specify a "trail" — either a fixed dollar amount or a percentage — and the stop follows the stock's highest price. It never moves backward. This lets profits run while still providing downside protection.
Every order you place has a time-in-force instruction that tells the market how long the order should stay active.
| Code | Name | Behavior | Use Case |
|---|---|---|---|
| DAY | Day Order | Expires at end of trading day if unfilled | Default for most orders. Use unless you have a specific reason not to. |
| GTC | Good Till Canceled | Stays active until filled or you cancel (brokers typically cap at 60–180 days) | Limit orders at prices you'd be happy to trade if reached anytime in the coming weeks. |
| IOC | Immediate or Cancel | Fills what it can immediately; cancels the rest | When you want partial fills but don't want the unfilled portion sitting in the book. |
| FOK | Fill or Kill | Fills entirely immediately or cancels entirely — no partial fills | When you need the full position size or nothing. Common in options. |
| MOO/MOC | Market on Open/Close | Executes at the market price at the opening/closing auction | When you want to participate in the opening or closing print specifically. |
A bracket order combines your entry, profit target, and stop-loss into a single package. When you enter a position, you simultaneously set a limit order to take profits at a higher price and a stop order to limit losses at a lower price. Whichever triggers first automatically cancels the other.
Bracket orders are the disciplined trader's best friend. They force you to define both your target and your exit before the trade begins, when you're thinking clearly — not after you're in the position and emotions are running.
We'll build on bracket orders and risk/reward math extensively in Module 7 (Risk Management). For now, understand the mechanics — Module 7 will teach you how to set the right prices.
Zero-commission trading doesn't mean free trading. The true cost of a trade is determined by execution quality, which includes the spread, slippage, and the speed at which your order is filled.
Price improvement. When your market order is filled at a better price than the displayed best bid/ask. Example: the best ask is $190.00 but your buy order fills at $189.99. Wholesale market makers (the ones paying for your order flow) are required to match or improve the displayed price.
Slippage. When your order fills at a worse price than expected. This happens most often with market orders during volatile moments — earnings releases, market opens, news events. If you place a market buy when the ask is $190.00 and you get filled at $190.15, you experienced $0.15 of slippage on 100 shares, that's $15 in real money.
Fill rate. For limit orders, what percentage of your orders actually execute? If you're consistently placing limits that never fill, you might be setting them too aggressively and missing opportunities.
Most brokers publish Rule 606 reports showing where they route orders and Rule 605 reports showing execution quality statistics. Check these periodically. If your broker consistently delivers poor execution, the "free" commissions may be costing you more than a broker that charges a small fee but provides better fills.
On August 24, 2015, the U.S. market opened sharply lower after a weekend of selling in Chinese markets. The Dow dropped over 1,000 points in the first minutes of trading. Many ETFs — including highly liquid ones tracking the S&P 500 — opened at prices 20–30% below their previous close, far below the actual value of their underlying holdings.
Traders who had stop-loss orders on these ETFs saw their stops trigger and convert to market orders — which then filled at the absurdly depressed opening prices. Some investors lost 20% in seconds on positions that recovered within minutes. The lesson: stop orders become market orders, and in illiquid or chaotic conditions, market orders can fill at horrifying prices.
This event reinforced why many experienced traders prefer stop-limit orders (accepting the risk of non-execution) or simply avoid holding stop orders over weekends and through known high-volatility events like earnings and major economic releases.
| Scenario | Recommended Order | Why |
|---|---|---|
| Routine entry on a liquid stock | Limit order | Price control with near-certain fill close to current price |
| You must exit immediately (emergency) | Market order | Guaranteed execution when speed matters more than price |
| Protecting a profitable position | Trailing stop | Locks in gains while allowing further upside |
| Setting a stop-loss on a swing trade | Stop order (liquid stocks) or Stop-limit (less liquid) | Balances automation with price protection |
| Defining risk and reward before entry | Bracket / OCO | Enforces discipline — target and stop set in advance |
| Buying on a dip that may or may not happen | GTC Limit order | Sits in the book for days/weeks waiting for your price |
| Participating in the opening or closing print | MOO or MOC | Executes at the specific auction price |
1. You want to buy 200 shares of TSLA, currently quoted at $245.00 bid / $245.05 ask. You want to pay no more than $244.50. What order do you use?
2. What is the primary risk of a stop-loss order?
3. In the AAPL order book example (Bid $189.98 / Ask $190.00), what is the spread and what does it represent?
4. You buy 100 shares at $50.00 and set a trailing stop with a $3.00 trail. The stock rises to $62.00, then falls. At what price does your stop trigger?
5. What does a bracket (OCO) order do?
6. Why do experienced traders generally prefer limit orders over market orders for routine entries?