How Orders Work

The short answer

An order is an instruction, and each type trades certainty of execution against certainty of price. A market order says "fill me now, at whatever price is available." A limit order says "fill me at this price or better, or not at all." Everything else is a variation on that one axis.

Choosing badly is not dramatic. It is a small, silent cost repeated on every trade, which is exactly the kind of cost that does the most damage over time.

The order book

Before the order types make sense, picture where they go. A venue keeps an order book: a list of resting buy orders below the current price and resting sell orders above it, each at a specific price with a specific size.

The best bid and best ask sit at the top of that book. Depth — how much size sits at each level — determines how far price moves when someone takes liquidity. A book with thin depth moves a long way on a modest order. A deep book absorbs the same order with barely a flicker.

Every order you place either adds to that book (rests there, waiting) or removes from it (executes immediately against what is already resting). That distinction is what maker and taker mean, and it is why the two are usually priced differently.

The four you need

Market order

"Fill me now." The order sweeps the book, taking the best available prices until it is filled.

• You are certain of execution.
• You are not certain of price.
• In a deep, liquid book at a quiet moment, the difference is negligible. In a thin book, or during a fast move, it can be large.

Market orders are the correct choice when getting out matters more than the exact level — which is a real situation, not a failure. They are the wrong default when neither of those pressures applies.

Limit order

"Fill me at this price or better." The order rests in the book until matched, cancelled, or expired.

• You are certain of price.
• You are not certain of execution — the market may never come to you, or may trade through your level without filling your full size.

A limit order that never fills costs nothing directly, but it has an opportunity cost: the move you were waiting for happened without you.

Stop order

"When price reaches X, submit a market order." A stop is a trigger, not a price guarantee.

This is the single most misunderstood point in this lesson. If you are long at 100 with a stop at 95, and the market gaps from 96 to 88 with nothing resting in between, your stop triggers at 95 and fills at 88. You did not do anything wrong. That is the instrument behaving exactly as specified.

Your actual loss can therefore exceed your planned loss. Position sizing that treats the stop as a hard floor is sizing on an assumption that does not always hold — a point the position size calculator spells out in its assumptions.

Stop-limit order

"When price reaches X, submit a limit order at Y." This adds price protection to the stop, at the cost of execution certainty.

The failure mode is the mirror image of the plain stop: in the fast move you were protecting against, price blows through your limit and the order does not fill at all. You wanted a smaller loss and you got an open position in a market moving against you.

There is no order type that gives you both certainties. That is not a gap in the product line; it is the nature of the trade-off.

Maker and taker

A resting limit order adds depth to the book — it is available for someone else to trade against. Venues generally want that, so they charge less for it, and some rebate it.

A market order, or a limit order priced so that it executes immediately, removes depth. Venues charge more for that.

Over a small number of trades the difference is trivial. Over hundreds, it is not. If your approach involves frequent entries and exits, the maker/taker split is a real input into whether it survives its own costs — which is what makes it a process decision rather than a detail.

Practical consequences

• Decide your order type as part of the plan, before the moment of clicking. Under pressure, everyone defaults to market.
• In a thin book, size down before you widen your stop. Slippage scales with the size you are pushing through the book.
• If you use stop-limits, know in advance what you will do if the limit does not fill.
• Check how your specific venue implements each type. Behaviour around triggers, partial fills and post-only flags is not standardised.