Glossary · PSX Mechanics
Order Book
A real-time electronic record of all buy and sell orders for a security on the exchange, showing prices and quantities at each level.
01—What is Order Book?
The definition — and what it means in practice.
An order book is a real-time electronic record of all buy and sell orders for a security on an exchange. It groups orders by price level and shows the quantity available at each level on the buy side (bids) and the sell side (asks). The best bid and best ask form the inside market, and the gap between them is the spread. As new orders arrive or get matched, the order book updates continuously.
For an investor, the order book helps you judge liquidity and potential execution quality before placing a trade. A deeper book (more quantity across levels) usually means you can trade without moving the price as much, while a thin book can lead to slippage. It also helps when choosing between a market order and a limit order, and when sizing your order so it fits typical available quantities.
If bids show 1,000 shares at Rs 50 and asks show 800 shares at Rs 51, the order book says you can likely sell near Rs 50 or buy near Rs 51.
- Shows live buy (bid) and sell (ask) orders, organised by price and quantity.
- Best bid and best ask are the most competitive prices currently available.
- Order book depth indicates liquidity and how much price may move when you trade.
- Thin order books increase the risk of slippage, especially with market orders.
- Large orders may fill in parts across multiple price levels.
02—How order book works on the PSX
The Pakistan-specific rules, conventions, and numbers.
On the Pakistan Stock Exchange (PSX), the order book is what you are effectively trading against during the pre-open session and continuous trading hours. As orders are added, cancelled, or matched, the displayed bid and ask levels change in real time. Because a standard board lot is typically 100 shares, the quantities you see in the book often cluster around multiples of that lot size.
PSX daily price limits (circuit breakers) for most equities are set around the previous close (LDCP). When prices approach those limits, the order book can become one-sided, with many orders stacked on one side and fewer on the other. In practical terms, that can affect whether your order is likely to execute quickly and at what price levels it may fill. The order book therefore becomes a key tool for understanding current supply and demand in the market.
03—Common misconceptions
Where investors most often get this wrong.
The order book shows all investor interest, so the price is always fair.
It only shows visible orders on the exchange at that moment. Sentiment can change quickly, and some participants may place, change, or cancel orders rapidly.
If there is quantity at a price, my whole order will fill at that price.
That quantity may be taken by other orders first, and large orders can fill across multiple levels. Execution depends on time priority and incoming flow.
A tight spread means there is no trading risk.
A tight spread can still sit on a thin book. If depth is low, even a modest order can move the price and create slippage.
04—Using order book on BSL
Where this term shows up across the platform — with live data.
- Check live demand and supply for a symbol on the Market view.
- Place and manage a limit order after understanding the Limit Order mechanics.
- Compare liquidity by scanning the Most Active list.
- Review which companies are in the KSE-100 Index when checking order book activity in large-cap names.
05—Frequently asked questions
What investors ask about order book on the PSX.
Frequently Asked Questions
It shows the currently available buy orders (bids) and sell orders (asks) for a particular security, grouped by price level with quantities. It updates as orders are entered, cancelled, or matched.
Yes, “market depth” is a common way platforms display the order book. It typically shows multiple bid and ask price levels and their quantities, not just the best bid and best ask.
A market order executes against the best available prices in the order book. If the book is thin, the order may consume several price levels, resulting in an average execution price different from the first displayed level.
Because the order book is live: participants add, modify, and cancel orders continuously during trading sessions. When trades occur, the best available quantities at the top levels can disappear and be replaced by the next levels.
No. Order book quantities are available only until they are matched or cancelled. Other orders may reach the market first, and execution follows the exchange’s matching and priority rules.
06—Related terms
Keep building the picture.
The highest price a buyer is willing to pay for a security. The gap between the bid and ask price is the spread and represents the cost of trading.
The lowest price a seller is willing to accept for a security. Also called the offer price. The difference between the ask and the bid price is called the spread.
The difference between the bid price and the ask price of a security. A narrow spread indicates high liquidity; a wide spread suggests lower liquidity and higher trading costs.
An instruction to buy or sell a security at a specific price or better. The order is only executed if the market reaches the specified price, giving the investor control over the execution price.
An instruction to buy or sell a security immediately at the best available current price. Guarantees execution but not the price at which the trade is filled.
The ease with which a security can be bought or sold without significantly affecting its price. High-liquidity stocks have large trading volumes and narrow bid-ask spreads.
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