Understanding the Order Book: Basic Concepts
Binance futures · BTCUSDT · FlowMap Chart
The order book is the list of buy and sell limits waiting on Binance BTCUSDT. Each row is a price and the size sitting there. A candle does not show that list. The book does.
This note is the basic layout: two sides, the best prices, the gap between them, and what happens when someone hits. The picture below is that layout.
Two sides
Bids are buy limits. The trader names a price and a size, and the order waits. They want to buy there, not higher.
Asks are sell limits. Same idea on the other side. They want to sell there, not lower.
Bids sit at or below the last price. Asks sit at or above it. The highest bid and the lowest ask are the two prices closest to the next trade.
Best bid, best ask, and the spread
The best bid is the highest price someone is willing to buy at right now. The best ask is the lowest price someone is willing to sell at.
The spread is best ask minus best bid. On BTCUSDT that gap is often one tick, the smallest price step on the contract. A wider gap means nothing is sitting in between. Crossing from one side to the other costs you that gap.
On FlowMap Chart those two prices are the BBO (best bid and offer). The live ladder next to price is COB, the current order book.
Size at each price
A price with no size is an empty step. A price with size is a row. The number is how much is waiting, in BTC on this contract, not dollars.
Several orders can sit at the same price. The book usually shows them as one row: that price, and the size added up. You do not see each trader. You see the pile.
A big pile can slow a push. It is still just orders. The people who placed them can pull the size before anyone hits it. A thick row is not a wall that has to hold.
Limits wait. Market orders hit.
A limit order joins the book and waits. A buy limit below the best ask sits with the bids. A sell limit above the best bid sits with the asks. Waiting size is what the book is made of.
A market order does not wait. A market buy lifts the ask: it buys from the sell limits already sitting, starting at the best ask. A market sell hits the bid: it sells into the buy limits, starting at the best bid.
If the size at that first price is smaller than the market order, the rest walks to the next price. That is how one order can print through several rows and move the price you see.
The course version is limit vs market.
Who is in front
Better price goes first. Among bids, the higher price is in front. Among asks, the lower price is in front. That is why the best bid and best ask are the next trade.
At the same price, the order that arrived earlier is in front of the ones that came later. You cannot see that queue on a normal book. You only see the size added together.
Thick book, thin book
Depth is how much size is sitting, at the best prices and further away. A thick book has size at many prices. A thin book has little size, so a market order walks further and the price jumps more.
The spread is the gap at the touch. Depth is the size behind it. A tight spread with no size behind it can still jump. A wide spread is already telling you the two sides are far apart.
The book keeps changing
The list updates when someone joins, pulls, or hits. A new limit adds a row, or adds size to a row that is already there. A cancel takes size away. A market order takes size away by trading it.
Two candles can look the same while the book did not. One minute the bids stayed and got hit. The next minute the bids pulled and price fell through a thin book. The candle keeps the high and the low. The book is what changed in between.
What you are looking at
COB on FlowMap Chart is this book, live, on Binance futures BTCUSDT. Bar length is size at that price right now. Heat to the left is where size was sitting earlier. Heat is history. COB is now.
A closed candle still cannot show either one. That split is in what BTCUSDT candles don’t show. The layer names are in how to read the FlowMap Chart on BTCUSDT.
What this picture is not
Not a setup. Not a promise that a big bid will hold. Not every Bitcoin exchange. Binance is one book. Other venues have their own bids and asks.
It is the visible limits on Binance futures BTCUSDT. Size that pulls before it trades never becomes a print. For learning only — not financial advice.
Watch the live order book on Desk or on FlowMap Chart · BTCUSDT.