Pressing buy in a brokerage app feels instant, but there is a small market behind that button, and the way you place the order decides what price you actually get. A few minutes on the mechanics saves real money over a lifetime of trades.
Stocks trade on exchanges and other venues where buyers post bids and sellers post asks. The highest bid and lowest ask form the quote. When you buy at market, you pay the ask; when you sell at market, you receive the bid. The gap between them, the spread, is a cost you pay on every round trip.
What to look at
- Bid: the highest price someone will pay right now. Ask: the lowest price someone will sell for. Spread = ask − bid.
- Market order: fills immediately at the best available price. Fast, but you do not control the price.
- Limit order: fills only at your price or better. You control the price, but it may not fill.
- Stop order: becomes a market order once a trigger price trades. Useful for exits, but it can fill well below the stop in a fast move.
- Regular US hours are 9:30am to 4:00pm Eastern. Pre-market and after-hours trading is thinner, with wider spreads.
The shape of it
Round-trip cost ≈ Spread + Commissions + Price impact
An example
A small company is quoted $9.90 bid, $10.10 ask. A market buy fills at $10.10 and an immediate market sell at $9.90: a 2% loss before the stock has moved at all. A limit buy at $10.00 might avoid half of that.
Where people go wrong
Market orders on thinly traded stocks, or at the open after big news, can fill far from the last price you saw. Use limit orders whenever the spread is wide or the stock is moving fast.
None of this decides whether a stock is a good investment. It decides how much of your return you hand away getting in and out. For long-term investors in liquid stocks the cost is small; for frequent traders in small stocks it can be the whole edge.