A price on a screen is a summary, and like most summaries it hides the thing that matters. The number you see is generally the price of the most recent trade, or the best price currently available for a small quantity. Neither is a promise about what you will pay.
What the book actually is
An order book is two queues. On one side, offers to buy at various prices, sorted with the highest first. On the other, offers to sell, sorted with the lowest first. The gap between the best of each is the spread, and no trade happens inside it.
When you place an order that demands immediate execution, it does not transact at one price. It consumes the queue: the best price first, then the next, then the next, until your quantity is filled. Your result is the weighted average of everything it touched.
Why size is not linear
This is the part that surprises people. Doubling your order size does not double your cost of trading; it usually more than doubles it, because the quantity available tends to thin as you move away from the mid-price. The first slice fills close to the quote. The last slice can fill materially worse.
The practical consequence is that “the price” is a function of how much you are trading. Two participants can transact in the same asset within the same second and achieve genuinely different prices, without anything unfair having happened.
Two order types, two different risks
An order that demands immediate execution accepts whatever price the book offers. You are certain to trade, and uncertain what you will pay.
An order that specifies a price and waits accepts the reverse. You know your price, and you do not know whether you will trade at all. Neither is safer in general — they trade one uncertainty for the other, and which one you should prefer depends entirely on which one would hurt you more.
The mechanism people notice too late
Because a resting order is a commitment visible to others, a book is not a passive object. Participants adjust their resting orders continuously in response to what they see. Depth that appears to be present can be withdrawn faster than an order can reach it, particularly when the market is moving.
This is not a conspiracy; it is what happens when everyone is managing risk in a market that never closes. But it does mean that a screenshot of a deep book is weaker evidence than it looks.
What to take from it
Read the quoted price as an approximation for small size and nothing more. Assume your effective price will be worse than the quote and get worse with size. Expect that to be most true when the market is most active. None of this argues for or against any trade — it is simply how the machinery works.
For the fundamentals see how to read a crypto chart and the glossary entry on the order book. Live prices for the assets we track are on the markets page. Not financial advice.