Because news arrives while the market is shut. Orders accumulate overnight and the opening auction sets a single price that clears them, so the adjustment happens in one step rather than gradually.
Regular trading covers six and a half hours of a twenty-four hour day. Companies report results, regulators decide, and events happen in the remaining hours.
When the market reopens, the price has to account for everything that happened since it closed. There is no path of trades in between — hence a jump.
Orders placed overnight are collected and matched in an auction that finds the single price at which the most shares can trade. That price becomes the open.
This is why a gap is not a series of trades you could have participated in. It is one clearing price, and the first regular-session print is already on the far side of it.
A gap on a scheduled event such as an earnings release is a re-pricing on new information. A gap on no visible news is more often an order-book effect and is more frequently retraced.
The size relative to the name's ordinary daily range says more than the absolute percentage — a 3% gap is unremarkable in a stock that routinely moves 4%.
By definition, the gap has already happened before regular trading begins. Reading a headline at 9:31 means reading it after the adjustment.
The session that follows is a separate question from the gap: what the rest of the day does, starting from the new price.
One question, no account: what did you come here for, and did you find it? A person reads these, and they shape what gets written next.
From StockMotive — the honest why behind every market move. Educational information only, not investment advice.