The screen said 40,000. You bought, and the fill came back at 40,036 average. Nothing went wrong, no one moved the price against you, and the number on the screen was accurate when you read it.
It’s the most common surprise in trading and it has a completely mechanical explanation, which is that the quoted price was the price of the first unit available and you bought more than one unit.
That’s the kind of thing this site is about. Not what to trade, not when, not whether — the plumbing. How a limit order book is structured and what the numbers on it mean. What each order type actually guarantees, and the harder question of what it doesn’t: a limit order can guarantee your price or your execution, never both, and a stop order guarantees neither. Where slippage comes from and how it differs from the impact your own order has on the book. What a round trip costs once the maker-taker model is included, which is usually more than the headline number. And the funding rate on perpetuals — a payment that moves between traders rather than to the venue, and the reason a contract with no expiry can still track a spot price.
None of this has a direction. It’s true whether you think an asset is going up or down, and it’s the same machinery whether you place one order a year or ten thousand a day.
Nothing here is financial advice, and there are no predictions of any kind — not about an asset, not about the market, not hedged into an aside. No venues are named, no products recommended. Mechanisms only, with the failure mode of each stated alongside it, because a mechanism described without its failure mode is a sales pitch.
Latest posts
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What an order book is
Two sorted queues of unexecuted intentions, and a matching rule. Almost everything surprising about execution follows from that structure.
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Order types and what each guarantees
You can guarantee your price or guarantee your execution, never both. Every order type is a different position on that single trade-off.
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Slippage and market impact
One is the gap between the price you expected and the price you got. The other is the part of that gap your own order caused. They are not the same.
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Maker, taker, and what you actually pay
The headline fee is one of at least four costs in a round trip. The others are the spread, market impact, and the difference between the two fee rates.
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Perpetuals and the funding rate
A futures contract with no expiry needs something to keep it near spot. Funding is that mechanism: a payment between traders, not to the venue.