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'''[[Order book]]''' is the continuously updated electronic record of all outstanding buy and sell orders for a financial instrument maintained by a [[stock exchange]], trading venue, or electronic trading system. The order book organizes orders according to price and, where applicable, time priority, enabling buyers and sellers to interact through a transparent and structured trading process. It is one of the fundamental components of modern electronic financial markets and plays a central role in [[price discovery]], [[market liquidity]], and [[market microstructure]].

An order book contains information about pending purchase orders (bids) and sale orders (asks or offers), including the prices and quantities at which market participants are willing to trade. As new orders are submitted, modified, cancelled, or executed, the order book is updated in real time. Trading occurs when compatible buy and sell orders are matched according to the rules established by the exchange or trading venue.

Order books are widely used across equity, bond, commodity, foreign exchange, derivatives, and cryptocurrency markets. Advances in electronic trading have transformed the order book from a manually maintained record into a highly automated system capable of processing millions of orders each day. Because it reflects current market demand and supply, the order book provides valuable information regarding trading interest, market depth, and short-term price dynamics.

The structure and operation of order books vary among markets. Some trading venues display the full order book publicly, while others reveal only limited information or incorporate [[hidden liquidity]] through non-displayed orders and [[Dark pool|dark pools]]. Regardless of the specific implementation, the order book remains one of the principal mechanisms through which financial markets facilitate efficient trading.

== History ==
The earliest organized stock exchanges maintained order books manually through brokers and exchange officials who recorded buy and sell interests during trading sessions.

The transition to computerized trading during the late twentieth century enabled electronic order books capable of continuously recording and matching orders automatically. These systems substantially improved trading speed, transparency, and operational efficiency.

The widespread adoption of electronic trading platforms, algorithmic trading, and low-latency communication networks has made electronic order books the standard mechanism for securities trading in most developed financial markets.

== Objectives ==
The principal objectives of an order book are to:

* organize outstanding trading orders;
* facilitate efficient order matching;
* support [[price discovery]];
* improve [[market liquidity]];
* increase market transparency;
* enable orderly trading.

These objectives contribute to the efficient operation of financial markets.

== Structure ==
A typical order book consists of two principal sections.

=== Bid side ===
The bid side contains outstanding buy orders arranged in descending order of price.

Each entry generally includes:

* bid price;
* order quantity;
* order priority.

The highest bid represents the maximum price currently offered by buyers.

=== Ask side ===
The ask side contains outstanding sell orders arranged in ascending order of price.

Each entry generally includes:

* ask price;
* order quantity;
* order priority.

The lowest ask represents the minimum price at which sellers are currently willing to transact.

== Example ==
A simplified order book may appear as follows:
{| class="wikitable"
!Bid quantity
!Bid price (US$)
!Ask price (US$)
!Ask quantity
|-
|500
|49.95
|50.05
|400
|-
|800
|49.90
|50.10
|600
|-
|1,200
|49.85
|50.15
|900
|}
In this example:

* the highest bid is '''US$49.95''';
* the lowest ask is '''US$50.05''';
* the [[bid-ask spread]] equals '''US$0.10'''.

== Order matching ==
Most electronic trading systems automatically match compatible buy and sell orders.

Matching generally follows:

* price priority;
* time priority.

Orders offering better prices receive execution priority, while orders at identical prices are generally executed according to the time at which they were entered.

== Order types ==
Several types of orders may be recorded in an order book.

Common order types include:

* market orders;
* limit orders;
* stop orders;
* stop-limit orders;
* iceberg orders;
* reserve orders.

Some non-displayed orders may not appear fully in the visible order book.

== Market depth ==
The order book provides information regarding market depth.

Market depth refers to:

* the number of buy orders;
* the number of sell orders;
* available trading quantities;
* price levels surrounding the current market price.

Greater market depth generally indicates stronger [[market liquidity]].

== Relationship with price discovery ==
The order book is one of the primary mechanisms supporting [[price discovery]].

As buyers and sellers submit new orders or modify existing ones, the balance between demand and supply changes continuously, influencing market prices.

Executed transactions establish new market prices that reflect prevailing trading conditions.

== Relationship with market liquidity ==
The order book provides a visible representation of [[market liquidity]].

Indicators of liquidity include:

* order quantities;
* market depth;
* [[bid-ask spread]];
* order flow.

A deep and active order book generally enables more efficient execution of trades.

== Relationship with algorithmic trading ==
[[Algorithmic trading]] systems interact continuously with electronic order books.

Algorithms commonly perform activities such as:

* submitting orders;
* modifying orders;
* cancelling orders;
* monitoring market depth;
* responding to changing market conditions.

Modern algorithmic trading depends heavily upon real-time order book information.

== Relationship with hidden liquidity ==
Not all trading interest is visible within the public order book.

[[Hidden liquidity]] may arise from:

* hidden orders;
* iceberg orders;
* reserve orders;
* [[dark pool]] trading.

Consequently, the visible order book may not represent the full amount of executable liquidity available in the market.

== Applications ==
Order books are widely used in:

* equity markets;
* bond markets;
* commodity exchanges;
* foreign exchange markets;
* derivatives markets;
* cryptocurrency exchanges.

They also support market surveillance, execution algorithms, and quantitative trading research.

== Advantages ==
Order books provide several important advantages.

They improve market transparency, facilitate efficient order matching, enhance price discovery, support market liquidity, reduce transaction costs, and enable fair execution based on objective priority rules.

Electronic order books also allow market participants to monitor current trading conditions in real time.

== Limitations ==
Despite their importance, order books have certain limitations.

Displayed orders may be cancelled before execution, and visible liquidity may not represent total executable liquidity because of hidden orders and dark trading. Large institutional orders may also be fragmented into numerous smaller orders, making true trading intentions difficult to identify.

Furthermore, rapidly changing market conditions may cause order book information to become outdated within fractions of a second in highly active markets.

== Importance ==
The order book is one of the core components of modern electronic financial markets. By recording and organizing outstanding trading interest, it enables efficient order matching, supports price discovery, improves market liquidity, and promotes transparent and orderly trading. Its widespread adoption has fundamentally transformed securities trading and remains essential to the operation of contemporary capital markets.

== See also ==

* [[Market order]]
* [[Limit order]]
* [[Bid-ask spread]]
* [[Market liquidity]]
* [[Price discovery]]
* [[Market microstructure]]
* [[Algorithmic trading]]
* [[Hidden liquidity]]
* [[Dark pool]]
* [[Stock exchange]]