Stock exchange

A stock exchange is an organized marketplace where stocks of public companies are bought and sold. The major exchanges — the New York Stock Exchange (NYSE), Nasdaq, the London Stock Exchange (LSE), and others — are highly regulated venues with listing requirements, trading hours, surveillance systems, and circuit breakers that halt trading during extreme moves. Trading is now almost entirely electronic, routed through order books and matching engines, though the image of the trading floor persists.

What a stock exchange does

A stock exchange is a venue—today, almost entirely electronic—that brings together buyers and sellers of stocks and executes trades. When you enter a buy order for 100 shares of Apple through your broker, your order is routed to the exchange, matched with a sell order, and executed. The execution typically happens in milliseconds. The exchange does not buy or sell itself; it is a middleman that matches counterparties and ensures fair, orderly trading.

To trade on an exchange, a company must be listed. Listing requires meeting financial thresholds (minimum market capitalization, earnings, assets), governance standards (independent board, audit committee), and disclosure rules (regular financial reporting). These requirements ensure that only established, regulated companies can trade publicly, which protects investors and lends credibility to the market.

Major exchanges are highly regulated. In the US, the Securities and Exchange Commission (SEC) oversees the markets; stock exchanges also enforce their own rules, including trading halts for material news, surveillance for market manipulation, and automatic circuit breakers that halt trading during extreme moves.

The evolution from floor to screen

Before the 1990s, stock exchanges had physical trading floors where traders in colored jackets shouted orders, negotiated prices, and executed trades. The New York Stock Exchange floor, in lower Manhattan, is still iconic and still operates, but the vast majority of trading is now electronic. Nasdaq, founded in 1971, was the first electronic stock market and has always been screen-based.

The shift to electronic trading dramatically increased speed and reduced costs. A trade that once took minutes and involved multiple intermediaries now happens in microseconds. Bid-ask spreads (the difference between what you can buy for and sell for) have narrowed. Transaction costs have fallen to near zero for retail investors, and market depth and liquidity have increased.

The flip side: modern electronic markets are fragmented. In addition to the primary exchanges (NYSE, Nasdaq), there are dozens of alternative trading venues called electronic communications networks (ECNs) and dark pools where trades happen off-exchange. Some of this fragmentation aids liquidity and price competition; some of it enables front-running and other abuses.

Listing standards and why they matter

To get a stock listed on a major exchange, a public company must clear hurdles. The NYSE and Nasdaq have different standards, but both typically require:

• Minimum market capitalization (often $100M+)
• Minimum earnings (or revenue, for newer companies)
• Minimum number of public shareholders
• A independent board of directors
• An audit committee
• Regular financial reporting (10-K, 10-Q)

These standards are meant to ensure that companies listed are real, regulated, and transparent. A company that fails to meet standards can be delisted—its shares are removed from the exchange and trade over-the-counter (OTC) or not at all. Delisting is humiliating and typically reduces the company’s valuation, because institutional investors often cannot hold OTC shares, and the price discovery is worse.

Primary and secondary listings

Most public companies have a primary listing on one exchange (say, Nasdaq). Some large international companies also list on a foreign exchange as a secondary listing. For example, a Chinese company might have a primary listing in Hong Kong and a secondary listing in New York. Secondary listings raise capital and increase visibility but impose additional compliance burdens.

Circuit breakers and trading halts

Stock exchanges have automatic safeguards to prevent panic selling and flash crashes. The US stock market has circuit breakers that halt all trading if the S&P 500 index falls 7%, 13%, or 20% in a single day. The first two triggers a 15-minute halt; the 20% drop halts trading for the rest of the day.

Additionally, an exchange can halt trading in an individual stock if there is a material news announcement (a merger, a bankruptcy, an executive resignation), pending release of quarterly earnings, or if there is an unusual pattern suggesting market manipulation or a technical malfunction. These halts are brief and are meant to give the market time to digest information or let systems recover.

Primary vs. secondary markets

The “stock market” actually comprises two distinct markets:

Primary market: Where stocks are first issued. When a company does an initial public offering (IPO), it sells new shares directly to investors through an underwriter, raising capital for the company. This is the primary market—money flows to the company.

Secondary market: Where already-issued stocks change hands. When you buy Apple stock on Nasdaq, you are buying from another investor, not from Apple. No money flows to Apple. The secondary market is where the vast majority of trading occurs, and it is what most people mean by “the stock market.” Exchanges operate both the primary market (IPOs are often listed here first) and the secondary market (ongoing trading).

Fragmentation and dark pools

The term “stock exchange” once meant a single, centralized marketplace. Today, there are many venues:

• Exchanges: NYSE, Nasdaq, and others, regulated and transparent.
• ECNs: Electronic Communications Networks like EDGX, MEMX, and IEX, which are regulated but smaller and less visible.
• Dark pools: Private trading venues operated by investment banks, brokers, or independent operators, where institutional investors can trade large blocks with less public information.

Dark pools reduce market impact (slippage) for large trades but reduce transparency. The SEC regulates them but does not require them to publish all trade data publicly. Some dark pools have been accused of front-running (trading ahead of customer orders or leaking information), though the largest ones are now heavily monitored.

For a typical retail investor, all of this fragmentation is irrelevant—your broker routes your order to the best available venue, and you get filled. For large institutional traders and market makers, the choice of venue is crucial.

See also