A limit order sets a maximum price for a purchase or a minimum price for a sale, but it never guarantees that the trade will actually happen. This is the single most important distinction to remember before placing one: this type of order protects the price, not the execution.
What the official definition of a limit order says
According to the investor bulletin published by the SEC, a limit order is an order to buy or sell a stock at a specific price or better. A buy limit order can only be executed at the limit price or lower, and a sell limit order can only be executed at the limit price or higher. The SEC explicitly states that a limit order is never guaranteed to be executed: it only triggers if the market price actually reaches the specified limit price.
This wording deserves to be read twice. It does not say the order will “probably” execute or will execute “in most cases”: it says there is no guarantee, full stop. If the market never touches your price, your order remains pending, potentially indefinitely, until it is cancelled or expires according to your broker’s rules.
Why the bid-ask spread can prevent the trigger from firing
To understand why a limit price can be missed even when the market seems to be approaching it, you need to look at the order book. The glossary from the American investor protection agency defines the bid as the highest price a buyer is currently willing to pay, and the ask as the lowest price a seller will accept. The SEC states in its fact sheet on the spread that the ask is almost always higher than the bid, with this gap serving as a source of compensation for market makers.
This detail changes everything for a buy limit order. If you place a buy limit order at a price sitting between the current bid and ask, it will only trigger if the market falls all the way to that exact level, not just if it visually approaches it on a chart. The definition of the bid price notes that this price moves continuously: a gap that looks negligible at first glance can be enough to leave a limit order unfilled for an entire session.
A fictional worked example to illustrate the mechanism
Here is a fully fictional example, built solely to illustrate the reasoning, with no connection to any real asset or market. Imagine an asset whose displayed bid is 100.00 and ask is 100.10, a fictional spread of 0.10. A trader places a buy limit order at 99.95, below the current bid, trying to buy below the current market price.
In this fictional scenario, as long as the bid does not fall to 99.95, the order remains pending: the displayed price could fluctuate between 99.98 and 100.05 for several minutes without ever triggering execution, precisely because the definition of a buy limit order only allows execution at the limit price or lower. If the market eventually touches 99.95, the order may execute at that level or at an even better price, but nothing guaranteed that level would be reached that day.
What a limit order does not cover
The sources cited here document the mechanics of price and spread, not execution speed, available liquidity at a given level, or how a market behaves during a sharp move. A limit order also does not protect against the total absence of a counterparty if no one is willing to trade at that price. For the separate question of trigger price versus execution price on a stop order, the article on stop orders and stop-limit orders covers a different mechanism, not to be confused with a standard limit order.
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