Reading the orderbook

Why this lesson exists

The orderbook is the most information-dense object on your screen. Almost no retail trader reads it well.

Charts tell you what happened. The orderbook tells you what's available right now — who is willing to buy, who is willing to sell, at what prices, in what size. Get fluent with it and every entry, exit, and stop placement gets sharper. Stay illiterate and you'll keep wondering why your fills are worse than the screen price you saw.

This lesson is the prerequisite for the entire Pattern Lab. Every pattern you'll practise is grounded in price — but the orderbook is grounded in liquidity, and the difference between a clean breakout and a head fake almost always shows up in the book first.

Bid, ask, and the spread

The orderbook has two sides.

• Bids are resting buy orders. Each bid is a trader saying "I'll buy this size at this price or lower." Bids stack downward from the best (highest) price.
• Asks are resting sell orders. Each ask is a trader saying "I'll sell this size at this price or higher." Asks stack upward from the best (lowest) price.

The best bid and best ask are the highest price someone will buy at and the lowest price someone will sell at, respectively. The gap between them is the spread.

spread = best_ask − best_bid

A tight spread means the market is liquid: buyers and sellers are clustered close together. A wide spread means liquidity is thin: there's a meaningful gap between the most aggressive buyer and the most aggressive seller, and crossing it costs you.

On BTC perp during the US session — say 21:30 IST on a Wednesday — the spread on Lighter is typically well under $2 with several million dollars of depth on each side. The same book at 23:00 IST on a Friday after US close can show a $10+ spread with a fraction of the depth. Same asset, same venue, very different friction cost.

The mid-mark and why xtree uses it

The mid-mark is the simple average of the best bid and the best ask.

mid_mark = (best_bid + best_ask) / 2

It's not the price of any actual trade. It's a synthetic reference — the fair midpoint between the most aggressive buyer and seller right now.

xtree uses mid-mark for every equity calculation that touches your account: unrealised P&L, MLL floor checks, DLL accounting, take-profit and stop-loss triggers. It does not use last-trade price, which is what most platforms display by default.

Why? Because last-trade can be a stale or one-off aggression print. Imagine the book is bid ₹85,00,000 / ask ₹85,00,200, and a single small market buy lifts the offer to ₹85,00,800. Last-trade is now ₹85,00,800. But the real, current fair value — what you'd actually be able to close at — is the mid-mark of ₹85,00,100.

If xtree used last-trade for your equity, a single thin print could push you toward an MLL breach you didn't deserve. Using mid-mark means the platform tracks the actual current fair value, not a noisy print. This matters more than it sounds; on volatile days, the difference between last-trade and mid can be a couple of basis points repeatedly throughout the session.

Depth: what's behind the best price

Looking at only the best bid and best ask is like reading only the first line of a paragraph. The depth behind those prices is what tells you whether the price is robust or fragile.

A deep book has substantial size sitting at and near the best prices. If the best bid shows 50 BTC and the next 5 price levels each show another 20+ BTC, then a 5 BTC market sell barely moves the market — it eats through the best bid and stops. The price is "supported" in the literal sense: there's real money willing to buy below.

A thin book has small size at the best, and the levels behind it are sparse. A 5 BTC market sell in a thin book can drop the price 0.3–0.5% in a single order because each level holds tiny size and the order eats through several of them.

On xtree's orderbook panel, depth is what the size column shows. Read it before you size your position. A 0.5 BTC entry in a deep book is invisible; the same 0.5 BTC in a thin Friday-evening book might move the market 5 bps against you on entry alone.

This is the structural argument for limit orders on thin books: a market order eats the book, a limit order sits inside it.

Reading the imbalance — gently

Imbalance is the ratio of total bid size to total ask size within some price range (say, 10 bps from mid). An imbalance of 3:1 on the bid side means there's three times as much resting buy demand as sell supply nearby.

Retail traders treat imbalance as a directional signal: lots of bids = price going up, lots of asks = price going down. This is wrong often enough to be dangerous.

The day after BTC's $73,777 peak in March 2024, the book repeatedly showed 3:1 bid-side imbalance as price drifted from $73K down to $66K over the following week. Retail traders going long on the "bullish imbalance" got run over. The resting bids were spoofed or pulled the moment price approached them — they were never real intent to buy.

Imbalance is information, not direction. A persistent imbalance that holds through several test prints might be meaningful. A snapshot imbalance is just noise. Treat the book as one input among many; never as the deciding one.

Decimal grouping for thin markets

xtree's orderbook lets you change the price grouping (the tick size at which liquidity is aggregated). On BTC, the default grouping is fine because BTC ticks tightly. On thinner markets — SOL perps, the xCOMMODITIES (xGOLD, xSILVER, xOIL) — liquidity can be scattered across many small ticks, making the book look thinner than it is.

Grouping by a larger tick aggregates that liquidity into visible clusters. Suddenly you can see that what looked like a single small ask is actually part of a 200-contract wall ₹50 above. Those visible clusters are usually where stops cluster too — and where reversal candles tend to print.

Switch the grouping on any thin market before deciding the book is thin. The book might be deeper than the default tick is showing you.

Worked example: a tight book vs a thin one

Same instrument, same price, different times.

Wednesday 21:30 IST, BTC perp on Lighter (US session)

Asks:
85,00,500   12.3 BTC
85,00,300    8.7 BTC
85,00,200   18.4 BTC   ← best ask

Bids:
85,00,000   22.1 BTC   ← best bid
84,99,800   15.9 BTC
84,99,600   10.8 BTC

Spread: ₹200 (~2 bps)
Depth (5 levels each side): ~88 BTC total

A 1 BTC market order moves the price within the best level. Negligible slippage. Mid-mark is solid.

Friday 23:30 IST, same book

Asks:
85,03,000    1.1 BTC
85,02,400    0.8 BTC
85,01,800    0.6 BTC   ← best ask

Bids:
85,00,200    0.9 BTC   ← best bid
84,99,400    0.5 BTC
84,98,600    0.7 BTC

Spread: ₹1,600 (~19 bps)
Depth (5 levels each side): ~5 BTC total

A 1 BTC market buy now eats through three levels of asks and fills at a weighted average closer to ₹85,02,200 — a real ₹400 of slippage on top of an already-wide spread.

Same trader, same intent, same screen. The friction cost is roughly 10× higher in the second case. The book told them that before they clicked.

Common misunderstanding

"The bid side is twice as deep as the ask side, so the price is going up."

Sometimes. More often, no. Resting orders are a snapshot of intent at a single moment. Intent can be pulled in microseconds. Large bids that sit for a while and absorb sell pressure (the price approaches them and they don't disappear) are meaningful — they're real buyers defending a level. Large bids that vanish the instant price approaches them were never real bids.

The way to tell the difference is to watch the book over time, not in a single snapshot. A bid that absorbs ten consecutive market sells without thinning out is real demand. A bid that disappears every time price comes close is a spoof or an algo testing the market — useful only as a contrarian signal at best.

This is why the orderbook is a tool, not an oracle. It rewards patience; it punishes shortcuts.

Recap

• The book has two sides: bids (resting buys) and asks (resting sells). The spread is the gap between best bid and best ask.
• Mid-mark = (best_bid + best_ask) / 2. xtree uses it for every equity calculation because it's harder to game than last-trade.
• Depth behind the best price is what determines slippage on market orders. Thin books punish market orders.
• Imbalance is information, not direction. A 3:1 bid imbalance is not a buy signal.
• Decimal grouping aggregates liquidity into visible clusters on thin markets — change it before judging depth.

Next up: Reading candlestick charts — OHLC mechanics, what wicks tell you, and the timeframe choice that decides how often you trade.

Also worth reading once you've got the basics: Breakout strategies — the orderbook is where you confirm whether a breakout has real flow behind it or is about to head-fake. And the MLL lesson connects the mid-mark concept here directly to how your account-ending floor is computed.

Test yourself

Next lesson: Reading candlestick charts — OHLC, wicks, timeframes, and what candles do and don't tell you about what's coming.