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There is no such thing as "the price". There are always two.
Your chart draws one line, and that line is a convenience. At every moment a real market has a price you can buy at and a different, lower price you can sell at. Everything in order flow is built on knowing which of those two prices a trade happened at.
01 — The two prices
Bid, ask, and the spread
- The bid is the best price you can sell into right now. Someone is bidding for your contracts.
- The ask is the best price you can buy from right now. Someone is offering contracts to you.
- The spread is the distance between them — the built-in cost of demanding immediacy.
In ES the spread is one tick (0.25) almost all of the time, because the market is deep enough that someone is always willing to stand on both sides. In NQ it is also usually one tick, but the tick is worth more and the size resting at each level is thinner — which is a large part of why NQ moves faster.
Think about it like this
A currency exchange booth posts two numbers: what they will buy your euros for, and what they will sell you euros for. The difference is how they get paid for standing there ready to trade. The order book is the same idea, except the booth is thousands of competing participants and the spread gets competed down to almost nothing.
02 — The skill
Classifying who was aggressive
Here is the important part, and it is the hinge the rest of the course swings on.
When a trade prints, the exchange tells you the price and the size. From that, plus the state of the book, you can work out which side crossed the spread:
- A trade printing at the ask means a buyer reached up and took it. Classified as aggressive buying.
- A trade printing at the bid means a seller reached down and hit it. Classified as aggressive selling.
This is the raw material. Every footprint chart, every delta number and every CVD line you will ever look at is just this classification, counted up and arranged differently.
| Time | Price | Size | Aggressor |
|---|---|---|---|
| 09:31:04 | 4512.25 | 40 | at ask — buyer |
| 09:31:04 | 4512.25 | 115 | at ask — buyer |
| 09:31:05 | 4512.00 | 22 | at bid — seller |
| 09:31:05 | 4512.25 | 57 | at ask — buyer |
| 09:31:06 | 4512.50 | 90 | at ask — buyer |
324 contracts traded in three seconds. 302 were buyers lifting offers, 22 were sellers hitting bids — and price ticked from 4512.25 to 4512.50. Aggression, and the movement it produced. Both halves matter.
The core idea
Volume tells you how much traded. Bid/ask classification tells you who was impatient. The second question carries the information about intent, and it is invisible on a standard candlestick chart.
03 — The nuance
Resting size is a promise, not a fact
Beginners discover the order book, see a 2,000-lot bid sitting three ticks below, and conclude price cannot go through it. Then price goes through it instantly and they cannot understand why.
Two reasons:
- Orders can be cancelled A resting limit order is a stated intention, not a commitment. It can be pulled in microseconds — and often is, the moment real aggression shows up. Size that vanishes when tested was never support.
- Displayed size is not all size Iceberg orders show a small amount and refill as they get filled. A level that looks like 200 lots can absorb 3,000. You often only discover it by watching how much trades there without price moving.
Common mistake
Trading the depth of market as if it were a support and resistance map — "there's a big bid there, I'll buy against it."
What is resting in the book is the least reliable information in order flow, because it is the easiest to fake and the cheapest to withdraw. What actually traded, and what price did while it traded, is the durable signal. Watch fills, not intentions.
04 — Putting it together
A practical example
NQ is approaching yesterday's high. You are watching the tape.
- Prints are landing at the ask, repeatedly, in 30–90 lot clips. Buyers are paying up.
- Price advances four ticks over about twenty seconds.
- Then prints keep hitting the ask — another 800 contracts — and price stops advancing.
Nothing about the classification changed. Buyers are still the aggressive side. But the relationship between aggression and movement broke. The first 400 contracts bought four ticks. The next 800 bought zero.
You cannot see that on a candle. On the chart it is one green bar with a wick. On the tape it is the single most informative thing that happened in that minute.
Key takeaways
- Every market has two live prices: the bid (where you can sell) and the ask (where you can buy). The gap is the spread.
- Trades printing at the ask are classified as aggressive buying; at the bid as aggressive selling.
- That classification is the raw input for delta, CVD, footprint and everything that follows.
- Resting book size is weak evidence — it can be pulled or hidden. Executed volume is strong evidence.
- Always read aggression together with the price movement it did or did not produce.
◆ Where this is going
Counting every print by hand does not scale.
You now know how a single trade gets classified. The next lesson takes thousands of those classifications and compresses them into one number you can read in real time.
Continue to Delta →