Limit order vs market order

An order is an instruction to your broker, which the broker routes to an exchange. The exchange runs the matching engine the previous page described; your order is what the engine reads when it decides who trades with whom. Two specifications cover almost every retail trade. A market order says fill me right now, at whatever price the book offers. A limit order says fill me only at this price or better, otherwise wait. Everything else (stop orders, trailing stops, fill-or-kill rules) is a refinement of those two ideas. The first decision a retail account makes on every trade is which of the two to use.

A market order prioritises certainty of fill over certainty of price. The order rushes to the top of the opposite side of the book and trades against the best available quote. On a liquid US large-cap that quote is usually pennies away from the midpoint; on an illiquid LATAM small-cap it can be a percent or more away. The cost is called slippage: the gap between the price you saw before clicking and the price you actually got. Slippage is silent on tight-spread names and noisy on thin-volume names. The market order does not know which is which; it simply takes whatever sits at the top of the book.

A limit order prioritises certainty of price over certainty of fill. You name the price you are willing to pay (for a buy) or accept (for a sell), and the order sits in the book until a counterparty meets that price or improves on it. The risk is that no one ever does, and your order expires unfilled. The benefit is that when it does fill, it fills at exactly your price or better. A limit order can also be marketable: if your buy limit is at or above the current best ask, the exchange treats it like a market order against the prevailing offer. Most trade-button defaults in retail brokers vary on this and it pays to know which your account uses.

The choice between order types is roughly free of consequence on liquid US large-caps and has real cost on illiquid LATAM-listed names. A market buy on a Brazilian B3 small-cap or a Mexican BMV mid-cap can move the price 1% to 3% just from your own buying pressure, because the book has fewer resting orders to absorb the flow. The same trade on AAPL or MSFT moves through a book deep enough that the slippage lands in pennies. Effective spreads are also wider for LATAM retail trading via international brokers because of currency conversion plus execution-cost stack on top of the underlying spread. Limit orders surface that cost up front; market orders absorb it silently.

Three threads pull this together. First, the order type only makes sense in the context of the matching engine the venue runs. The book reads your instruction; the type tells the book what flexibility you allow. Second, the underlying claim is the same act either way: buying a piece of a company happens at whatever price the engine matches, regardless of how you specified it. Third, slippage is part of the all-in cost of investing the same way fund fees are. A 1% slippage on a $10,000 trade costs $100; if you trade four times a year that is 0.4% of returns lost to the wrapper, on top of any TER. For LATAM small-cap exposure especially, the default-to-limit-order discipline saves real money.