Reading the order book: market depth, queues and what the bids are telling you

KARACHI: Every share on the Pakistan Stock Exchange has a hidden crowd standing behind its price: buyers queued at descending prices, sellers queued at ascending ones, each with a quantity and a place in line. That crowd is the order book, and the window onto it — market depth — is the most informative, and most misread, screen in a trading app. Beginners stare at it hoping it predicts the future. Professionals use it for something humbler and more valuable: to know what trading this share, today, will actually cost, and how their own order will behave. This guide teaches that humbler, profitable reading.

What you are looking at: anatomy of a depth screen

A market depth display shows the best several levels of the central order book described in our guide to how PSX order matching works. On one side, bids: prices buyers are offering, highest first, each with the total shares queued at that price and often the number of separate orders composing it. On the other, offers (asks): prices sellers demand, lowest first, with their quantities. The top line on each side is the best bid and best offer; the difference between them is the spread. Below each top level, prices step away in the share's tick increments — bids descending, offers ascending — each level a queue operating on price-time priority.

Run your eye over an illustrative book, invented for teaching, prices in rupees. A liquid large company might show: bids of 12,000 shares at 842.50, 25,000 at 842.00, 40,000 at 841.50; offers of 9,000 at 843.00, 18,000 at 843.50, 30,000 at 844.00. The spread is fifty paisa on an Rs 843 share — tight, a fraction of a percent. Contrast a neglected small company: bids of 200 shares at 18.00, then nothing until 500 at 16.50; offers of 300 at 21.00, then 1,000 at 24.00. The spread there is three rupees on a share whose last trade was near nineteen — an enormous percentage gap — and there are price levels with nobody home at all. These two books describe two different practical realities that no price chart conveys: in the first, you can buy or sell a meaningful quantity near the quoted price, in seconds. In the second, the "price" is almost a rumour; any real order moves it violently.

The four things the book genuinely tells you

First, liquidity — honestly measured. Forget a share's reputation; count the shares actually queued near the top. Depth within, say, one percent of the best prices is the quantity you could trade without moving the price much. If your intended order is larger than the visible depth near the top, you are not trading at the displayed price; you are trading through several prices, and the book shows you in advance roughly where your average fill will land.

Second, your immediate transaction cost. If you must trade now, as a buyer you cross the spread and pay the best offer; as a seller you receive the best bid. The spread is therefore a cost as real as commission — on the small-company example above, buying at the offer and selling at the bid moments apart would lose a double-digit percentage with the market standing still. Aktzien is being built around low-cost trading and plans to make costs like these visible, because investors who can see the spread stop donating it carelessly: they use limit orders, trade patiently, or decline trades whose spread swallows the idea. Our earlier piece on market makers and liquidity explains who profits from wide spreads and why some PSX shares barely trade.

Third, where your limit order will sit. Place a bid at a level already crowded with 50,000 shares and, by time priority, you are behind all of them; the level must trade that entire quantity before your first share fills. Place it one tick above and you lead the market — at the cost of paying a tick more if filled. The book converts "should I bid 100 or 100.20?" from superstition into arithmetic.

Fourth, short-term pressure — with strong caveats. A book heavy with bids near the top and thin offers above does suggest buyers are keener at this moment, and rapid changes — offers being pulled, bids stacking — accompany news and momentum. Day traders read these shifts professionally. But this is the book's least reliable message for everyone else, for reasons the next section details.

The five ways the book misleads the unwary

• Orders are intentions, not commitments. A resting order can be cancelled in a blink. A huge bid that makes a share look supported can vanish the moment price approaches it; strategies that display orders mainly to be seen exist, which is one reason market surveillance watches order behaviour. Treat displayed size as a statement that may be revised, not a wall.
• You see only the top of the iceberg. Apps show a limited number of levels. Beyond them may sit large orders at further prices — or nothing. A book that looks deep for 10,000 shares may be empty for 50,000.
• Hidden and conditional interest exists off-screen. Not every intention is displayed in the visible book at all times; large holders act through brokers, in pieces, and across days. The book is a snapshot of declared, momentary intent, not a census of everyone who cares about the share.
• One moment is not a trend. Books at the open, near the close, around announcements and at price limits behave abnormally. A depth reading taken during a panic or a halt resumption describes that storm, not the share's normal weather.
• At the price limit, the book lies by omission. When a share is locked at its upper or lower circuit limit, the queue on one side balloons — thousands of buyers at the ceiling, no sellers — which reads as strength or despair but mainly records that the rules will not let price move further today. Our learn guide to circuit breakers explains the mechanics.

A worked reading: two orders, two decisions

Suppose, as a teaching example, you want to buy 5,000 shares of a mid-sized company. The book shows offers: 1,500 shares at Rs 210.00, 2,000 at Rs 210.50, 4,000 at Rs 211.00, 6,000 at Rs 212.00, and recent trades are printing at Rs 210. A market order to buy 5,000 will take all three top levels and 1,500 shares of the fourth, filling at an average of roughly Rs 210.70 — about a third of a percent above the last price, before commission. A limit order at Rs 210.00 joins the bid side instead: if the best bid is already Rs 209.90 with 8,000 shares queued, your Rs 210.00 bid becomes the new best bid and the next sellers must come to you — but they may never come, and the share may run away upwards unfilled while you saved seventy paisa a share. Neither choice is "correct" in the abstract. The book has told you the price of immediacy (about Rs 0.70 a share, Rs 3,500 on the order, in this illustration) and the price of patience (execution risk). Making that trade-off consciously, order by order, is the entire professional use of market depth.

Now invert it as a seller of an illiquid share: 2,000 shares to sell; the book shows bids of 300 shares at Rs 45, 500 at Rs 43, then a void to Rs 38. A market order will average somewhere in the low forties and may print the share down sharply, frightening other holders — some of the "crashes" in tiny PSX companies are one impatient seller meeting an empty book. A patient limit offer at a realistic level, or selling in small tranches over days, may recover several rupees per share. In illiquid names, how you trade routinely matters more than what you trade, and the order book is where that truth is visible before it is expensive.

Depth, volume and the tape: using the book with its companions

The book is one of three live witnesses; cross-examine it with the other two. Trades (the tape) show what actually executed — prices, sizes, times. A level that repeatedly trades without its queue shrinking is being replenished; a level that trades once and disappears was real but finished. Volume and value traded for the day, compared with the share's typical day, tell you whether today's book is being written by unusual interest. On the PSX, where many listed companies trade only a few thousand shares on a quiet day, these comparisons are stark: a share whose whole visible bid side totals less than your intended order is telling you, politely and in advance, to resize, to wait, or to use the services of a broker who can work the order. Also learn your own platform's refresh behaviour — depth shown by broker apps derives from the exchange feed, and during fast markets any display is a slightly delayed photograph of a moving crowd.

One more structural point: depth differs dramatically by company because it reflects the free float — shares actually available to trade — and the shareholder base's habits. A company mostly held by a sponsoring family and institutions that rarely trade will show a hollow book however large its market capitalisation. Index membership interacts here too: companies in major indices attract the continuous attention that thickens books, which is one practical reason index rebalancing, covered later in this series, moves prices — entering an index brings new, habitual buyers whose orders populate the book.

A beginner's routine for any PSX trade

• Before ordering, open the depth screen and note the spread in percentage terms. If it is wide, default to a limit order and patience.
• Compare your order size with the quantity queued at the top three levels on your side. If you dwarf them, expect to move the price; split the order or stage it.
• Choose your limit with the queue in mind: join a level, improve it by a tick, or cross the spread — each is a decision with a cost you can now name.
• After placing, watch executions, not just price: partial fills tell you how the queue ahead of you is clearing.
• Never infer tomorrow from the book. It answers "what will trading cost me now?" superbly and "where is the price going?" hardly at all.

Trading hours, depth display formats, tick sizes and order types are set by the PSX and brokers and change over time; confirm today's specifics in current PSX sources and your broker's platform documentation. This article is educational content, not financial advice. But the reading skill itself does not expire: wherever there is a queue of bids and offers, the investor who reads it calmly will trade better than the one who reads it hopefully.

A word about the large orders you will occasionally see parked far from the action — a bid for a huge quantity several rupees below the market, an offer equally far above. Sometimes these are genuine instructions from investors content to wait for a dislocation; sometimes they are the visible edge of an institution building or exiting a position in stages; sometimes they are simply old intentions nobody has cancelled. The book gives you no way to know which, and guessing is a parlour game, not analysis. What such orders do reliably is mark the prices at which size has declared an interest, which can matter on the day the market actually travels there. Note them, by all means — then return to the top of the book, where the spread, the near depth and the queue in front of your own order are the three facts that will determine what your next trade costs you. Mastery of the order book is mostly the discipline of attending to those three facts and declining to be hypnotised by the rest of the screen.

Key takeaway: the order book tells you liquidity, the true cost of trading now, and where your order will queue — it does not predict prices, because displayed orders can vanish, levels can hide, and patience is often worth more than size.