How a Stock Exchange Really Works

An exchange is a regulated electronic marketplace. Its matching system brings compatible orders together, records executed trades and publishes prices. It also runs the opening and closing auctions, enforces trading rules, and has listing and monitoring duties. This lesson follows the path of one normal cash-market trade.

Open a broker app and search for ITC. You may see an NSE quote and a BSE quote. The company is the same, but the exchange you pick decides which order book receives your instruction. Mr. Chartist's broker, Sahi (Aaritya Broking Private Limited, NSE member 90375), gives him access; NSE runs the matching system.

The company name identifies the business; the trading symbol identifies the security on one exchange. On NSE, ITC Ltd trades under the symbol ITC in the EQ series. Confirm the exchange and the security before ordering. A familiar name does not remove investment risk.

Trading on the exchange is anonymous. When Mr. Chartist buys, he never learns who sold, and the seller never learns who bought. In this lesson the other side is simply an unknown seller. NSE and BSE keep separate order books for the same listed share.

Because the lists are separate, the best prices on the two exchanges can differ a little at one moment. On 21 October 2026, for example, ITC's day high was ₹264.85 on NSE and ₹264.75 on BSE. Neither exchange is wrong; each book holds different waiting orders. An exchange is also not your broker, it does not hold your shares, and it cannot tell you whether a price is fair or a company is sound.

An order is your instruction to buy or sell a certain number of shares. The order book is the live list of all such orders for one share that are still waiting to be matched, sorted by price. Every share has its own book, and a new order either joins the list or meets an order already waiting. Understanding this one list explains most of how prices move on the screen.

A market-depth screen is the practical version of this list. Buy orders are bids and sell orders are asks. The best bid is the highest buying price; the best ask is the lowest selling price. Quantities tell you how many shares are available at a level, not whether the company is a good investment.

The gap between the best bid and the best ask is the spread. In the ITC book below we label the sellers S1 to S4 and the buyers B1 to B4, with the best price first on each side. The price levels sit inside ITC's real range on 19 October 2026 (₹259.95 to ₹263.90), and prices move in steps of ₹0.05. The order sizes and the 9:20 am times are illustrations.

Here the best bid is ₹262.65 and the best ask is ₹262.70. The spread is 262.70 − 262.65, which is ₹0.05, one price step. As a share of the price, it is 0.05 ÷ 262.70, or about 0.019%. The spread is the small gap between buyers and sellers, and it is the first thing to look at when you open a quote.

The waiting orders total 200 shares on the bid side and 150 shares on the ask side. Nothing trades while the best bid stays below the best ask, because no buyer and seller agree yet. A trade happens only when a new order meets the best price on the other side. Apps usually show only a few levels of the book, not all of it.

When a new order arrives, the exchange fills the best price first. Among orders waiting at the same price, the one that arrived earlier goes first. This rule is called price-time priority. NSE numbers and time-stamps every order when it arrives and keeps the book sorted by price, then by time. The rule looks only at price and arrival time, never at who sent the order.

Imagine that your ITC buy order joins other orders at the same price. A seller arrives with fewer shares than the total waiting demand. The exchange needs a consistent way to choose which orders receive those shares: better price first, then the earlier exchange timestamp at an equal price.

The timestamp that matters is when the exchange receives the order, not when you tap a button. Your connection, broker checks and transmission time come before that timestamp. A larger investor does not jump ahead of a smaller investor at the same price.

Take the buy orders B1, B2 and B3 from the book. B1 wants 30 shares at ₹262.65 and was received at 9:20:02. B2 wants 20 shares at ₹262.65 and was received at 9:20:05. B3 wants 100 shares at ₹262.60 and was received at 9:20:01. B3 arrived first of the three, but at a lower price.

Now a seller sells 40 shares at ₹262.65. B3 waits because its price is lower, even though its time is earliest. B1 and B2 share the best price, so B1 goes first and is filled fully with 30 shares, worth 30 × 262.65 = ₹7,879.50. B2 gets the remaining 10 shares, worth 10 × 262.65 = ₹2,626.50, and keeps waiting for its other 10. The total is ₹10,506.00, which is 40 × 262.65.

This is why a limit order, an order at your own chosen price, may not fill even when the price touches your level. Whether it fills depends on how many shares are ahead of you at that price. Small orders and large orders follow the same rule, so the first order received does not always fill first.

A market order seeks execution against waiting opposite-side orders, with no chosen price. A limit order can execute at your limit or better; any unfilled balance may wait, depending on validity. A stop-loss waits in a separate stop-loss book until its trigger is reached, then enters the active matching book.

For a buyer, a limit is the maximum acceptable price; for a seller, it is the minimum acceptable price. A limit order can execute at once if compatible orders already exist; it does not always have to wait. A market order can receive several prices, or only a partial fill when too few shares are waiting.

Take the ITC book again. A market buy for 50 shares fills 10 shares at ₹262.70, then 20 at ₹262.75 and then 20 at ₹262.80, because it walks up through the waiting sellers. The total is 2,627.00 + 5,255.00 + 5,256.00, which is ₹13,138.00. The average price is ₹262.76, so the quick trade cost ₹3.00 more than 50 shares at ₹262.70 (₹13,135.00).

A limit buy for 50 shares at ₹262.75 behaves differently. It fills 10 shares at ₹262.70 and 20 shares at ₹262.75, for ₹7,882.00 in all. The other 20 shares cannot be filled without paying more than ₹262.75, so they wait in the book as the new best bid. This order never pays more than the limit you set, but a fill is not certain.

A stop-loss limit order becomes an active limit order after its trigger. It controls the acceptable price but can remain unfilled. A stop-loss market order seeks execution at available prices after triggering; neither the fill nor the final price is guaranteed. Check which order types and validity conditions your broker and segment support.

Say Mr. Chartist holds 10 ITC shares and places a stop-loss sell with trigger ₹255.00 and limit ₹254.50 (an illustration). That is below the day's real low of ₹259.95 and above the past-year low of ₹253.15 (8 Oct 2026). On NSE, a stop-loss sell is released to the matching book only when the last traded price falls to or below the trigger. If the price then falls below ₹254.50 with no buyer at or above that limit, the order may stay unfilled.

After you tap Buy, three separate bodies each keep one record of your trade. The exchange keeps the trade itself. The clearing corporation keeps the record of who owes what. The depository keeps the record of who owns what. Each record answers a different question, and no single body holds all three answers.

Three practical records help you trace the trade: the executed-order record or contract note, the settlement obligation, and your demat holding. The app can show an execution before the share credit appears. Matching and settlement are different stages.

The earlier lesson, What Is the Stock Market, named these hops one by one. Here we look only at the three records they leave behind. None of these bodies judges whether your purchase was wise. Each one only writes down a fact about the trade and passes the work to the next body in the chain.

Say Mr. Chartist buys 10 ITC shares on NSE at ₹262.70, the real opening price on Monday 19 October 2026 (the quantity is an illustration). That is 10 × 262.70 = ₹2,627.00 before charges. Under T+1 the trade settles on the next settlement working day. Tuesday 20 October was Dussehra, an NSE trading and clearing holiday, so this trade settles on Wednesday 21 October.

NSE records the executed quantity and price. NSE Clearing works through members to calculate funds and securities obligations, which are final by 9:00 am on the settlement day. The money comes from Mr. Chartist's HDFC Bank account through Sahi. The shares reach his demat account through the depository; Sahi is a CDSL depository participant. These are linked processes with different responsibilities.

Before continuous trading starts at 9:15 am, NSE runs a short auction called the pre-open session. It collects orders and finds one opening price for each share. Nothing trades until that price is found. Then ordinary trading begins, with orders matching one by one as they arrive.

Since 7 September 2026 the session runs like this. From 9:00 to 9:05 am you can enter, change or cancel limit and market orders. From 9:05 the window takes limit orders only, and it closes at a random moment in the last 2 minutes, between 9:08 and 9:10. Orders are matched and trades confirmed by 9:12, a buffer runs to 9:15, and then continuous trading starts. Stop-loss orders are not allowed, and trades done in pre-open cannot be cancelled.

On Monday 19 October 2026, ITC's previous close was ₹263.50 and the auction set its open at ₹262.70: 0.80 lower, or −0.30%. The table shows how such a price is found. NSE does not publish pre-open order sizes, so the quantities are illustrations, built so the answer lands on the real ₹262.70. For each price, count buyers willing to pay that price or more, and sellers willing to accept that price or less. The shares that can trade are the smaller number.

At ₹262.70, buy demand is 700 and sell supply is 600, so 600 shares can trade. The other prices allow 200, 400, 400 and 200. So ₹262.70 gives the largest matched quantity, and all 600 shares trade at that one price. The imbalance at ₹262.70 is 700 − 600 = 100 shares; across the whole illustrated book of 1,000 buys and 1,000 sells, 400 buy shares and 400 sell shares are left unmatched.

If several prices tie for the most shares, the rule picks the one with the smallest imbalance, and then the one nearest the previous close. If no price can be found, the first trade in normal trading becomes the open. Unmatched orders move into normal trading with their original time. The day can also end with an auction: for stocks in futures and options (F&O), such as ITC, a closing auction from 3:15 to 3:35 pm sets the closing price. It set ITC's ₹260.80 close on 19 October.

Liquidity means how easily a share can be bought or sold at about the same price. It is high when the spread is small and orders wait at several prices close together. Depth means how many shares wait at each price. A deep book can take a large order without the price jumping around.

Compare two snapshots. Snapshot A is the ITC book from section 2. Snapshot B is an assumed thin book at the same price level, a teaching condition, not a claim about ITC's usual liquidity. ITC traded 1,15,28,925 shares on NSE on 19 October 2026, so its real book is usually much deeper than snapshot B. A book can also change between the moment you look and the moment you trade.

In snapshot A, buy 10 shares at ₹262.70 for ₹2,627.00, then at once sell into the unchanged ₹262.65 bid for ₹2,626.50. The spread cost is 10 × ₹0.05 = ₹0.50, about 0.019% of ₹2,627.00. This isolates the spread: brokerage, taxes, other charges and changes in the book are left out.

In snapshot B, assume a ₹262.20 bid and a ₹262.70 ask, with only 100 shares at each best price. The spread is ₹0.50, or about 0.19% of the ₹262.70 ask: ten times snapshot A's spread. An order larger than the shares waiting at the best price reaches worse prices, which is why depth matters as well as the spread.

Buying 100 shares at ₹262.70 costs ₹26,270.00. Selling them at once into the unchanged ₹262.20 bid returns ₹26,220.00. The ₹50.00 difference is about 0.19% of the purchase value, before charges. These assumed conditions do not predict any company's future price.

Finally, a price on the screen does not mean you can sell at that price. If your order is larger than the shares waiting near the best price, it walks through the book, as in the section on order types. To read a quote on your own screen, see the lesson on reading a stock quote.

Clearing works out the money and shares each side owes after a trade. Settlement moves them. India's normal cash-market cycle is T+1: the next settlement working day after trade day T. Saturdays, Sundays, exchange holidays and bank holidays do not count. An optional T+0 segment also exists for eligible trades; this lesson's examples use T+1.

Here is a real calendar. Mr. Chartist's ITC trade happened on Monday 19 October 2026. Tuesday 20 October was Dussehra, a trading and clearing holiday on NSE. So T+1 was Wednesday 21 October: obligations were final by 9:00 am that day, and shares and money moved the same day. A trade on Friday 16 October settled on Monday 19 October, with no holiday in between.

Clearing can offset purchases against sales where the segment allows net settlement. This cuts the number of transfers without erasing the individual trade records. Trade-for-trade securities and some other cases are settled trade by trade, so netting is not a rule for every security.

Suppose the clients of one clearing member buy 1,000 ITC shares on a day and also sell 600 (the quantities are illustrations). Together that is 1,000 + 600, or 1,600 shares of trading. The net to settle is only 1,000 − 600, which is 400 shares. This is simplified, because real netting is done per member per share, but the idea is the same.

A broker app may show a holding before settlement completes. Confirm the real share credit in your demat transaction record or depository statement; an executed order alone does not prove delivery. The settlement calendar, holidays and market timings come in the lesson on settlement and market timings.

The clearing corporation acts as the central counterparty for accepted trades, through clearing members. It steps in as the buyer to every seller and the seller to every buyer, a process called novation. Retail investors normally deal through their broker and its member arrangements, not directly with the clearing corporation.

India's two equity clearing corporations are NSE Clearing Ltd and BSE Clearing Ltd (formerly Indian Clearing Corporation Ltd). Under interoperability, a clearing member can choose one of them to clear and settle its trades from more than one exchange. So choosing NSE or BSE does not, by itself, fix the clearing route.

Take Mr. Chartist's trade: 10 ITC shares at ₹262.70 from an unknown seller, a trade value of ₹2,627.00. In the simple picture, the clearing corporation stands between the buying and selling sides; in practice, the members carry the obligations. Novation removes the need to trust an unknown seller. It does not insure the share price.

Three things back this arrangement. Members, the brokers who trade through it, post margin up front. Gains and losses on open positions are settled each day. A settlement guarantee fund absorbs a default, up to its size: NSE Clearing reports a core fund of more than ₹2,800 crore against a minimum required corpus of ₹1,587 crore (its page updated 23 June 2025), and it has been rated AAA by CRISIL since 2008. These steps make completion of the trade far more dependable.

It is just as important to see what this does not cover. The clearing corporation completes the trade. It does not protect the price you paid, which stays at market risk, and it does not protect you if the company does badly. A broker misusing your money is a different matter with a different protection. If a seller fails to deliver, the steps in the next section apply.

For normal T+1 trades, a short delivery leads to a buy-in auction on T+1, after pay-out, with the auction settled on T+2. Shares the auction cannot buy are closed out in cash instead. The buyer does not pay for the seller's failure; the clearing rules decide the outcome.

A shortage is not the same as an unfilled order. The trade was already matched, but the selling side failed to deliver the shares. The clearing corporation follows its shortage process instead of simply deleting the trade.

Here is an illustration on ITC's real prices. Suppose the unknown seller's member fails to deliver the 10 shares sold at ₹262.70 on Monday 19 October. The shortage shows up at pay-in on Wednesday 21 October. NSE Clearing first debits the member at the valuation price, which is the settlement price of the trading day before pay-in: the average price of the last 30 minutes, across exchanges. We could not obtain ITC's 19 October settlement price, so we use the ₹260.80 close as a stand-in: 10 × 260.80 = ₹2,608.00.

The auction then runs on 21 October. Say it buys the shares at ₹264.00, a price inside ITC's real range that day (₹260.15 to ₹264.85); the auction price itself is an illustration. Because 264.00 is above the 260.80 valuation price, the member pays the difference, 10 × (264.00 − 260.80) = ₹32.00, plus charges. Mr. Chartist receives his shares on Thursday 22 October (T+2).

If the auction finds no seller, the trade is closed out in cash. The rate is the higher of two numbers. One is the highest price across the exchanges from the trade day to the auction day: for ITC from 19 to 21 October, NSE's highest was ₹264.85 (21 October), a real figure; BSE's high on 21 October was lower, at ₹264.75, and we use ₹264.85 here. The other is 20% above the settlement price on the auction day. Using the 21 October close of ₹262.00 as a stand-in, that is 262.00 × 1.20 = ₹314.40. The higher is ₹314.40, so for 10 shares the buyer would receive 314.40 × 10 = ₹3,144.00 in cash instead of the shares. Some special cases have their own rules.

Exchanges use price bands, operating ranges, market-wide circuit breakers and surveillance to keep trading orderly. They are different controls. On NSE, many shares have a fixed daily band of 2%, 5%, 10% or 20%. Shares that have their own futures and options have no fixed band; instead the exchange sets an operating range, 10% on NSE's published rule, to stop prices that are not genuine.

ITC is in futures and options, so it has no fixed daily band. Do not treat a 5% or 10% band from a classroom diagram as any share's actual limit. Check the current published band or range, the segment and the surveillance status for the share you are trading.

Market-wide circuit breakers watch the whole market. They trigger at 10%, 15% and 20% moves in the Nifty 50 or the Sensex, whichever is hit first, and they halt all equity and equity-derivative trading across the country. The effect depends on the time. A 10% move before 1:00 pm halts trading for 45 minutes; between 1:00 and 2:30 pm, for 15 minutes; at or after 2:30 pm, there is no halt. A 15% move before 1:00 pm halts trading for 1 hour 45 minutes; between 1:00 and 2:00 pm, for 45 minutes; on or after 2:00 pm, for the rest of the day. A 20% move at any time stops trading for the rest of the day. Each halt ends with a 15-minute pre-open session.

These halts are rare but real. On Friday 13 March 2020, the Nifty 50 hit its 10% lower circuit in early trade and trading stopped for 45 minutes, the first index circuit since May 2009. The Nifty had closed at 9,590.15 the day before and fell as low as 8,555.15, yet it closed the day at 9,955.20. Ten days later, on Monday 23 March 2020, the Sensex hit its 10% lower circuit and trading again stopped for 45 minutes. The Nifty closed that day at 7,610.25, down from 8,745.45, a fall of 12.98%.

The trigger levels are worked out each day from the previous day's index close. From the Nifty's close of 22,888.95 on 19 October 2026, the lower levels would be about 20,600 (10%), 19,456 (15%) and 18,311 (20%), our own arithmetic; the exchanges publish the official figures. Exchanges also keep surveillance lists, under SEBI's framework, for shares with unusual price or volume patterns. These lists are a form of watching, not predictions.

A circuit does not mean a share is safe or doomed. A lower circuit can mean no buyer exists at the permitted price, since a band slows a move but does not create buyers. Band lists, halt rules and surveillance stages come in the lesson on circuit limits and surveillance. None of these controls is advice about what to buy or sell.

Each body has its own job, and none guarantees that your trade was a good investment. The exchange matches orders by one rule; it does not set the price or hold shares. The broker takes your order and sends it; it does not match orders or set the price. Keeping the jobs separate is what makes the system dependable.

On a real quote, first confirm the company, security and exchange. Then read the bid, ask and available quantities. After ordering, check the execution record; after settlement, check the demat holding. Each screen answers a different question.

The clearing corporation completes the trade and stands in the middle, but it does not protect the price you paid. The depository records who owns the shares; it does not match or clear trades. SEBI makes and enforces the rules for all of them, but it does not place trades or promise gains.

Follow Mr. Chartist's 10-share ITC trade: Sahi routes the order, NSE matches it at ₹262.70 on Monday 19 October 2026, NSE Clearing works out ₹2,627.00 and 10 shares through the members, and on Wednesday 21 October the shares reach his demat account at CDSL. Alerts and account statements help you check each record.

The chain protects the process, which is the trade completing correctly. It does not protect the price. ITC closed that same day at ₹260.80, below his ₹262.70 price, which shows the price risk stays with the investor. Your shares sit in your demat account, not with your broker. Markets carry the risk of loss of capital.