Why companies appear in several stock exchanges?
"If I buy the one from NSY, is it the ""real"" Sinopec? No - you are buying an American Depository Receipt. Essentially some American bank or other entity holds a bunch of Sinopec stock and issues certificates to the American exchange that American investors can trade.  This insulates the American investors from the cost of international transactions.   The price of these ADRs should mimic the price of the underlying stock (including changes the currency exchange rate) otherwise an arbitrage opportunity would exist.  Other than that, the main difference between holding the ADR and the actual stock is that ADRs do not have voting rights. So if that is not important to you then for all intents and purposes trading the ADR would be the same as trading the underlying stock." "The case you are looking at is rather special, because the Chinese government for the longest time did not allow foreigners to invest in Chinese stocks. The ADRs explained in @DStanley's answer are a way around that restriction; recently there are some limited official ways,  In general, it is perfectly normal for a stock to appear on different exchanges, in different currencies, and it's all the ""real"" stock. Because remember: a stock exchange is really nothing more than a fancy place for people to buy and sell stocks. There is absolutely no reason why a specific stock should only be traded in one place. Companies that have decided to be publically tradeable generally want to be traded in as many exchanges as possible, because it makes the stock more liquid, which helps their shareholders. Individual exchanges have different requirements for a stock to be listed for trading there, some may even do it without the company's explicit approval."
Two reasons why I think that's irrelevant: First, if it was on 3/31/2012 (two other sources say it was actually 4/3/2012), why the big jump two trading days later? Second, the stock popped up from $3.10 to $4.11, then over the next several trading days fell right back to $3.12. If this were about the intrinsic value of the company, I'd expect the stock to retain some value. Typically the settlement price for a financial instrument (such as AAPL stock) underlying a derivative contract is determined from the average price of trading in that instrument during some short time window specified by the exchange offering the derivative.  (Read the fine print on your contract to learn the exact date and time of that settlement period.) Because it's in an exchange's best interest to appear as fair as possible, the exchange will in general pick a high-volume period of time -- such as the close of trading on the expiry date -- in which to determine the settlement price. Now, the expiry date/time may be different from the last time at which the option can be traded, which may be different from the underlying settlement time.  For example, most US equity options currently expire on the Saturday following the third Friday of the month, whereas they can last be traded at end-of-day on the third Friday of the month, and the settlement period may be at a slightly different time on the third Friday of the month.  (Again, read the contract to know for sure.)  Moreover, your broker may demand to know whether you plan to exercise the option at an even earlier date/time. So, to answer your question: After-hours trading can only affect the settlement price of an underlying instrument if the exchange in question decides that the settlement period should happen during after-hours trading.  But since no exchange that wants to stay in business would possibly do that, the answer is no. Contract expiry time, contract exercise time, final contract trading time, and underlying settlement time may all fall at different dates/times.  The important one for your question is settlement time. Good day! Did a little research by using oldest public company (Dutch East India Company, VOC, traded in Amsterdam Stock Exchange) as search criteria and found this lovely graph from http://www.businessinsider.com/rise-and-fall-of-united-east-india-2013-11?IR=T :  Why it is relevant? Below the image I found the source of data - Global Financial Data. I guess the answer to your question would be to go there: https://www.globalfinancialdata.com/index.html  Hope this helps and good luck in your search! Stock exchanges have been undergoing a period of consolidation for the past hundred years for the exact reasons you mentioned. The existence of digital trading, harmonized laws and regulations, and fewer relevant currencies have made it more practical for mergers and acquisitions between exchanges. Stock exchanges are most often times private companies that compete with other exchanges, so that also promotes the existence of many exchanges. NYSE started allowing four letter tickers around 2009.  NASDAQ allows 4-5 letter tickers.  I guess they'll keep increasing when / if needed.  Companies are allowed to change tickers, although there are costs.  Tickers in the US are assigned through a single entity.  Companies that are new need to take something that's open. http://www.wsj.com/articles/SB124296050986346159 I see that you're in Australia, but, since there aren't really that many options to deal with the problem that you mentioned, I'd guess that you'll ultimately do the same.  Not sure about how tickers are assigned there though. Originally, stocks were ownership in a company just like any other business- you expected to make a profit from your investment, which is what we call dividends to stock holders. Since these dividends had real value, the stock price was based on what this return rate was, factoring in what it might be expected to be in the future, etc.  Nowdays many companies never issue any dividends, so you have to consider the full value of the company and what benefit could be gained by another company if it were to acquire it. the market will likely adjust the share price to factor in what the value of the company might be to an acquirer.  But otherwise, some companies today trading at an astronimical price, and which nevers pays a dividend- chalk it up to market stupidity. In this investor'd mind, there is no logical reason for these prices, except based on the idea that someone else might pay you more for it later... for what reason? I can't figure it out. Take it back to it's roots and imagine pitching a new business idea to you uncle to invest in- it will make almost nothing compared to it's share price, and even what it does make it won't pay anything to him for his investment. Why wouldn't he just laugh at you? Every company has Stocks.   For the stocks to be traded via some stock exchange, the companies must follow the eligibility criteria and guidelines. Once done, these are then listed on the stock exchange and can be traded. The advantage [amongst others] of listing is liquidity and stocks can easily be bought and sold.  Some small companies or closely held companies may not want to list on stock exchange and hence are not traded. This does not mean they can't be bought and sold, they can be outside of the market, however the deals are complex and every deal has to be worked out. During the course of time a stock that is traded on a stock exchange, would either fail to meet the criteria or voluntarily choose not to be traded and follow the delisting process [either by stock exchange or by company]. After this the stocks are no longer traded on the exchange. They get a piece of existing and potential market for a relatively small cost. Even large players can't ignore new market entrants,as some of them tend to grow at a fascinating rate. If you buy a company withhuge potential to grow, you save yourself in the process and also make money long term.