What Is a Stock Exchange?
Michael Whitman
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Trading since 2017, full-time since 2020
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Published 2026-09-06
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Updated
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Reviewed
A stock exchange is a regulated venue that matches buy and sell orders under published rules. It sets listing standards for the companies traded on it, runs the opening and closing auctions, and publishes the prices and trades its own matching produces.
A stock exchange is a venue that matches buy and sell orders under rules it has registered with the Securities and Exchange Commission. In the United States it is also only one of the places a listed share can trade, and by Cboe’s count, in September 2026 it was the smaller one.
How it works
A stock exchange does two jobs. It matches orders, and it enforces rules about who may trade, what may be traded, and how. Strip away the history and the building, and what remains is a computer applying a published priority rule to a queue.
The matching rule is price first, then time. A better price goes ahead of a worse one. Among orders at the same price, the earlier one fills first. That is the same rule described on the order book page, and the book is the exchange as a trader meets it.
The second job is listing. An exchange sets requirements, such as a minimum share price, market value, number of shareholders, and governance and reporting standards, and companies that meet them may list there.
A listing is a floor, not an endorsement. It says a company clears a published bar. It says nothing about whether the current price is sensible, and companies fall below the standards and get delisted.
How an exchange gets registered
Federal law makes registration the price of operating. Section 5 of the Securities Exchange Act of 1934 (15 U.S.C. 78e, read on govinfo.gov on 3 October 2026) makes it unlawful for a broker, dealer or exchange to use an exchange to effect a securities trade unless that exchange is registered as a national securities exchange, or exempted because its volume is too small for registration to be practical.
Section 6 sets the bar. An exchange applies by filing its rules with the SEC (15 U.S.C. 78f(a)), and the Commission may register it only if it finds the exchange is organized and able to comply with the securities laws and to enforce its members’ compliance with those laws and its own rules (78f(b)(1)). Subject to limits set out in the same section, the rules must also let any registered broker or dealer become a member.
Rule changes go through the SEC too. Each exchange files proposed changes to its rules, and the SEC posts them for comment. Its National Securities Exchanges filings page, read on 3 October 2026, listed three releases dated 2 October alone, for Cboe BZX, IEX and MIAX, an options exchange.
The two auctions
A US stock session runs six and a half hours, from 9:30 a.m. to 4:00 p.m. Eastern time, and continuous trading is only part of it. The session opens with an auction and closes with another.
An auction collects orders and strikes a single price that lets the largest quantity trade. Everyone in it gets that one price. The listing exchange’s opening and closing auctions set the official open and close, two of the four prices on every daily candle.
That is why an opening price is not the first tick of a trend. It is a clearing price for the overnight interest, and it can sit well away from the previous close without a single continuous trade in between.
The closing auction is where index funds and benchmarked portfolios meet, because they must trade at the official close. Cboe’s figures show how lopsided that can get: on most days auctions are a small share of trading, and on a few days they are a large one (see the data below).
A worked example
Take 30 September 2026, the last session of the month. Cboe’s market-volume page for that day reports 18,724,868,643 shares traded across all US venues.
- On exchanges: 9,356,034,346 shares, 49.97% of the total.
- Off exchange, reported to FINRA’s trade reporting facilities: 9,368,834,297 shares, 50.03%.
- In exchange auctions: 1,750,913,123 shares, 9.35% of all shares that day, found by subtracting Cboe’s auction-excluded exchange total (7,605,121,223) from the full one.
So on an ordinary month-end day, roughly every other share traded never touched an exchange’s book, and close to one share in ten changed hands at a single auction price.
The original data
Cboe Global Markets publishes the share of US stock trading done at each venue. Its US equities market-volume pages and daily files were read on 3 October 2026: every session of each September from 2021 to 2026, and the daily file for the 22 sessions from 2 September to 2 October 2026. Cboe’s month-to-date figure on its 30 September page stops at 29 September, so each September here is added up from the daily files for all of that month’s sessions. Auctions are included unless stated.
In September 2026, an average of 16.99 billion shares traded a day over 21 sessions, split like this:
| venue group | share of shares traded |
|---|---|
| off exchange (FINRA trade reporting facilities) | 50.63% |
| NYSE’s five exchanges | 19.80% |
| Nasdaq’s three exchanges | 14.38% |
| Cboe’s four exchanges | 8.55% |
| Investors Exchange (IEX) | 4.19% |
| MEMX | 1.64% |
| MIAX Pearl | 0.61% |
| Texas Stock Exchange | 0.15% |
| 24X National Exchange | 0.04% |
| Long-Term Stock Exchange | 0.01% |
All 18 exchanges traded shares on every one of the 22 sessions, and together they handled 49.37% of shares in September. Measured by dollar value instead of shares, the off-exchange share was 47.75%.
The off-exchange share has grown. Each September, added up from Cboe’s daily figures for every session:
| September | off exchange, shares | off exchange, dollar value | shares a day |
|---|---|---|---|
| 2021 | 41.92% | 35.30% | 10.58 billion |
| 2022 | 41.66% | 39.01% | 11.40 billion |
| 2023 | 44.13% | 42.34% | 10.34 billion |
| 2024 | 47.47% | 44.55% | 11.86 billion |
| 2025 | 50.97% | 44.11% | 18.31 billion |
| 2026 | 50.63% | 47.75% | 16.99 billion |
Cboe’s own group shrank over the same five Septembers, from 13.57% of shares to 8.55%, and two of the nine exchange groups in 2026, the Texas Stock Exchange and 24X, had no figures in September 2021.
Auctions swing hard by day. Across the 22 sessions, exchange auctions carried a median of 7.47% of all shares traded. On Friday 18 September 2026, the third Friday of the month, they carried 28.54%, and it was the only day of the 22 when off-exchange trading fell below half, to 42.76%. The daily figures are in the off-exchange daily file and the September table in the venue share file.
In practice: one listing, many venues
The listing exchange is where the company is listed, not where every trade happens. A listed stock trades across all 18 exchanges and off exchange at once, and the best bid and offer is assembled from all of the exchanges’ quotes.
Off-exchange trades are reported fast, not late. FINRA Rule 6380A (read on finra.org on 3 October 2026) requires a firm reporting to the FINRA/Nasdaq trade reporting facility to send each trade “as soon as practicable but no later than 10 seconds after execution”, and trades reported later are marked late. Those reports feed the same consolidated volume a chart shows.
Who does the off-exchange trading is a separate question: dark pools run as alternative trading systems, and wholesalers that fill retail orders, often paid for through payment for order flow, both report through the same facilities. Cboe’s figures do not split the two.
Fee structures differ between venues and shape routing. Some pay for resting orders and charge for removing them; others do the reverse. Those fractions of a cent per share are invisible to a retail trader and decisive to a professional one.
When it fails
The exchange matches what can be matched, at whatever price is there. A market order sent into a thin book fills against the resting orders, at their prices. The machine does its job and the fill is still poor. A limit order away from the market does not fill at all.
Auctions are not immune either. An opening auction on light interest can strike a price that looks nothing like where the stock trades ten minutes later, and orders left in it get that price.
The rules can stop trading entirely. Halts for news, for volatility limits, or for market-wide circuit breakers all suspend matching. During a halt no order fills, including the protective order you were relying on.
Treating the listing as due diligence is a mistake. A company can meet every standard on the day it lists and be a poor investment throughout. The exchange checked a checklist; it did not form a view.
And an exchange does not hold your shares. Your broker does that, and the ownership record sits with a central depository. If a broker fails, the question is about the broker’s custody and the investor protection scheme, not about the exchange.
Related
The order book is the queue an exchange operates. Trading sessions is where the hours and their consequences are set out. And market makers are the participants who keep quotes on both sides of that queue through the day.
I size differently around the open and the close, because both are auctions rather than continuous trading. They are the two moments the whole market agrees to meet at once, and treating them as just more bars is how you end up with a fill you did not expect.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.