Stock market basics · Beginner

How Stock Exchanges Work

Exchanges sort orders by price and time and match whatever crosses. That single mechanism explains fills, slippage, halts and why OTC stocks behave so differently.

Last reviewed 15 September 2026 by the MarketPro research desk · Editorial policy

In short

A stock exchange is a matching engine: it sorts buy and sell orders by price, then by time, and pairs whatever crosses. The last trade becomes the quoted price. The exchange takes no view on value. It supplies the rules, the technology and the disclosure regime that make a share tradable between strangers, and the depth of its order book decides whether your order fills cleanly or moves the price against you.

What to remember

The quoted price is the output of matching, not a valuation. It reflects the last trade, which may have been 100 shares.
A large market order walks up the book: 5,000 shares against thin depth can fill 10 cents above the offer and cost $500 in slippage.
The real product an exchange sells is enforced disclosure. That is what OTC markets mostly lack.
Check average daily volume before sizing. Getting in is easy; getting out requires a buyer to be there on your day.

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An exchange is a matching engine, not a valuer

A stock exchange is an organised venue that pairs buy orders with sell orders under published rules. It does not decide what a company is worth, does not hold an opinion on it, and has no view on whether your order is sensible. It takes the orders that arrive, sorts them by price and then by time, and matches whatever crosses. The price you see quoted is the output of that sorting, nothing more.

Understanding this removes a whole class of confusion. When a stock falls 8% on no news, the exchange did not reprice it. Sellers arrived at prices low enough to meet the bids that were resting there, and the last of those trades became the number on your screen.

Walking a 5,000 share order through a real order book

The book is a queue of resting orders at each price. Suppose the sell side looks like this.

Price	Shares offered	Cumulative
$47.84	1,200	1,200
$47.86	800	2,000
$47.91	1,500	3,500
$48.05	2,600	6,100

Send a market order for 5,000 shares and you do not get $47.84. You take all 1,200 there, all 800 at $47.86, all 1,500 at $47.91, then 1,500 of the 2,600 sitting at $48.05. The average fill works out at about $47.94, roughly 10 cents above the quoted offer, and the screen now shows $48.05 as the last price. You moved the stock yourself, and paid for the privilege.

On 5,000 shares that ten-cent slippage is $500. On a liquid mega-cap with 40,000 shares resting at the top of the book, the same order would have filled entirely at $47.84 and moved nothing. Liquidity is not an abstraction. It is the depth of that table, and it is the difference between a clean fill and an expensive one.

What a listing forces a company to do

In exchange for access to public capital, a listed company accepts obligations that a private one does not carry.

Periodic disclosure. Audited annual accounts and, in most markets, quarterly results on a published timetable.
Material event reporting. A change that a reasonable investor would want to know about has to be announced to the whole market at once, not briefed to selected holders.
Continued listing standards. Minimum share price, minimum public float, governance requirements. Fail them and the company is moved down or delisted, which is why reverse splits exist.

This disclosure regime is the real product an exchange sells. It is what makes a share in a company you have never visited a thing a stranger is willing to buy from you at a known price within seconds.

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OTC markets and what you give up there

Not every stock trades on a main exchange. Over-the-counter markets run through dealer networks rather than a central book, and the listing bar is far lower. Some legitimate foreign companies trade there through depositary receipts. So do shell companies, pre-revenue ventures and businesses that failed an exchange standard.

The practical costs are consistent. Spreads are wider, often several percent rather than a few basis points. Depth is thin, so an ordinary-sized order moves the price. Disclosure may be minimal or absent. And exit liquidity is the real problem: getting in is easy because someone is selling, getting out requires a buyer who may not be there on the day you need one.

Three things the printed price does not tell you

The last trade is a fact about two people, not a valuation of the company. Specifically, it does not tell you the size that traded, which may have been 100 shares. It does not tell you whether you could transact there now, since the resting bid may be several cents lower. And it does not tell you whether the price is reasonable, only that it cleared.

Exchanges also have states that catch people out. Auctions at the open and close set prices through a different mechanism than continuous trading. Volatility halts pause a stock after an outsized move. Extended-hours sessions run on a fraction of normal depth, which is why an after-hours print following a market-moving release often looks nothing like the next morning’s open.

Trading the book instead of the headline number

Two habits follow directly. Before sending an order, look at the bid, the offer and the sizes attached to each, not just the last price. If your order is a meaningful fraction of what is resting, use a limit order and accept that you may not be filled rather than paying through several price levels for certainty.

And check average daily volume before you size a position. A rule many use is to keep an order under about 1% of a stock’s typical daily volume, so that exiting does not require the market to accommodate you. That constraint belongs in the sizing decision from the start, which is what the position size calculator and the wider risk framework are for.

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Written and reviewed by the MarketPro research desk

MarketPro is a trading-signal and trading-education app operated by Harajuku Holdings LTD (Cyprus). Our signal desk publishes and tracks every trade idea in-app, and every page here is checked against the app's live behaviour before publishing. Last reviewed 15 September 2026. How we produce signals and content · About MarketPro

FAQ

How Stock Exchanges Work: questions

How does a stock exchange decide the price of a share?

It does not decide anything. The exchange matches the highest resting buy order against the lowest resting sell order and publishes the price at which they trade. If sellers want out badly enough to hit lower and lower bids, the published price falls. Supply and demand set it; the exchange only records and enforces the process.

What is the difference between an exchange and an OTC market?

An exchange runs a central order book with published listing and disclosure standards that companies must keep meeting. OTC markets run through dealer networks with far lower requirements, so information may be sparse, spreads are usually much wider and depth is thin. The same order size that fills invisibly on a major exchange can move an OTC stock several percent.

Why did my market order fill at a worse price than the one I saw?

Because the price you saw applied to a limited number of shares. Once your order consumes everything at that level it continues to the next price up, and the next, until it is filled. The average fill is therefore worse than the top of the book. A limit order caps the price you accept, at the cost of possibly not being filled at all.

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