When you see a stock price move up or down, it is not random. Every price change is the result of actual buy and sell orders being matched in an order book. Understanding how this works demystifies price movement.
What an Order Book Is
An order book is a real-time list of all pending buy orders (bids) and sell orders (asks) for a specific stock, organized by price.
Bid side (buyers): Shows every pending buy order with the price buyers are willing to pay and the number of shares at each price level.
Ask side (sellers): Shows every pending sell order with the price sellers want and the number of shares available at each price level.
How Prices Move
The stock price moves when a buy order and a sell order agree on a price.
Price goes up when: There are more aggressive buyers than sellers. Buyers are willing to pay the asking price (or higher), consuming available sell orders and pushing the ask price up.
Price goes down when: There are more aggressive sellers than buyers. Sellers accept the bid price (or lower), consuming available buy orders and pushing the bid price down.
Bid-Ask Spread
The difference between the highest bid and the lowest ask is called the spread.
- Tight spread (e.g., $50.00 bid / $50.01 ask): Lots of liquidity. Easy to trade without giving up much on price. Common for large, actively traded stocks.
- Wide spread (e.g., $50.00 bid / $50.50 ask): Less liquidity. Trading costs more because you lose the spread amount on each transaction. Common for smaller, less liquid stocks.
Market Depth
Market depth refers to how many shares are available at each price level in the order book. Deep markets have large quantities at each price level, making it difficult for a single order to move the price significantly. Shallow markets have small quantities, meaning even modest orders can cause noticeable price changes.
This is why news about a small company can cause its stock to jump 10% while similar news about Apple might move it 0.5%. The order books are vastly different in depth.
Why This Matters for Investors
Limit orders make more sense when you understand the order book. You are essentially placing your order into the book at a specific price and waiting for someone to match it.
Market orders make more sense when you understand that you are simply accepting the best available price from the opposite side of the book.
Liquidity matters because it determines how easily you can enter and exit positions without significantly affecting the price.
The Bottom Line
Stock prices are not set by mysterious forces. They are set by the collective decisions of every buyer and seller, moment by moment, through the order book. Understanding this mechanism gives you a more grounded view of how markets actually function.
The Progressive Trailblazer focuses on helping you understand the fundamentals behind stock prices. Educational only. Not financial advice.