Where do online stock brokers get their real-time data from?
Generally Google gets their data,  directly from the exchanges (Nasdaq, NYSE). This is really expensive   -- tens of thousands of dollars a month just for the license from the exchange, and lots of telecom costs on top of that. As another answer started, this information comes straight from an exchange and generally costs a fortune . . . However things change: IEX, a new exchange, recently opened and they are offering real time bid/ask data for free. Here's the API description: https://www.iextrading.com/developer/ This data should be good for active securities, but for securities less actively traded the numbers might be stale.
They could have different quotes as there are more than a few pieces here.  Are you talking a Real Time Level II quote or just a delayed quote?  Delayed quotes could vary as different companies would be using different time points in their data.  You aren't specifying exactly what kind of quote from which system are you using here. The key to this question is how much of a pinpoint answer do you want and how prepared are you to pay for that kind of access to the automated trades happening?  Remember that there could well be more than a few trades happening each millisecond and thus latency is something to be very careful here, regardless of the exchange as long as we are talking about first-world stock exchanges where there are various automated systems being used for trading. Different market makers is just a possible piece of the equation here.  One could have the same market maker but if the timings are different,e.g. if one quote is at 2:30:30 and the other is at 2:30:29 there could be a difference given all the trades processed within that second, thus the question is how well can you get that split second total view of bids and asks for a stock.  You want to get all the outstanding orders which could be a non-trivial task. "And what exactly do I profit from the short? I understand it is the difference in the value of the stock. So if my initial investment was $4000 (200 * $20) and I bought it at $3800 (200 * $19) I profit from the difference, which is $200. Do I also receive back the extra $2000 I gave the bank to perform the trade? Either this is extremely poorly worded or you misunderstand the mechanics of a short position.  When you open a short position, your are expecting that the stock will decline from here.  In a short position you are borrowing shares you don't own and selling them.  If the price goes down you get to buy the same shares back for less money and return them to the person you borrowed from.  Your profit is the delta between the original sell price and the new lower buy price (less commissions and fees/interest).  Opening and closing a short position is two trades, a sell then a buy.  Just like a long trade there is no maximum holding period. If you place your order to sell (short) 200 shares at $19, your initial investment is $3,800.   In order to open your $3,800 short position your broker may require your account to have at least $5,700 (according to the 1.5 ratio in your question).  It's not advisable to open a short position this close to the ratio requirement.  Most brokers require a buffer in your account in case the stock goes up, because in a short trade if the stock goes up you're losing money.  If the stock goes up such that you've exhausted your buffer you'll receive what's known as a ""margin call"" where your broker either requires you to wire in more money or sell part or all of your position at a loss to avoid further losses.  And remember, you may be charged interest on the value of the shares you're borrowing. When you hold a position long your maximum loss is the money you put in; a position can only fall to zero (though you may owe interest or other fees if you're trading on margin).  When you hold a position short your maximum loss is unlimited; there's no limit to how high the value of something can go.  There are less risky ways to make short trades by using put options, but you should ensure that you have a firm grasp on what's happening before you use real money. The timing of the trades and execution of the trades is no different than when you take a plain vanilla long position.  You place your order, either market or limit or whatever, and it executes when your trade criteria occurs." I don't know what restrictions are put on the average employee at your company. In my case, we were told we were not permitted to either short the stock, or to trade in it options.  That said, I was successful shorting the exact number of shares I'd be buying at the 6 month close, the same day the purchase price would be set. I then requested transfer of the stock purchase to my broker where the long and short netted to zero.  The return isn't 15%, it's  100/85 or 17.6% for an average 3 months they have your money. So do the math on APR. (Higher if the stock has risen over 6 months and you get the lower price from 6 months prior.) My method was riskless, as far as I am concerned. I did this a dozen times. The stock itself was +/- 4% by the time the shares hit, so in the end it was an effort, mostly to sleep better. I agree with posts suggesting the non-zero risk of a 20% 4 day drop. Retail brokers and are generally not members of exchanges and would generally not be members of exchanges unless they are directly routing orders to those exchanges. Most retail brokers charging $7 are considered discount brokers and such brokers route order to Market Makers (who are members of the exchanges). All brokers and market makers must be members of FINRA and must pay FINRA registration and licensing fees.  Discount brokers also have operational costs which include the cost of their facilities, technology, clearing fees, regulation and human capital. Market makers will have the same costs but the cost of technology is probably much higher. Discount brokers will also have market data fees which they will have to pay to the exchanges for the right to show customer real time quotes.  Some of their fees can be offset through payment for order flow (POF) where market makers pay routing brokers a small fee for sending orders to them for execution. The practice of POF has actually allowed retail brokers to keep their costs lower but to to shrinking margins and spread market makers POF has significantly declined over the years.  Markets makers generally do not pass along Exchange access fees which are capped at $.003 (not .0035) to routing brokers.  Also note that The SEC and FINRA charges transactions fees. SEC fee for sales are generally passed along to customers and noted on trade confirms. FINRA TAF is born by the market makers and often subtracted from POF paid to routing firms. Other (full service brokers) charging higher commissions are charging for the added value of their brokers providing advice and expertise in helping investors with investment strategies. They will generally also have the same fees associated with membership of all the exchanges as they are also market makers subject to some of the list of cost mentioned above.  One point of note is that Market Making technology is quite sophisticates and very expensive.  It has driven most of wholesale market makers of the 90s into consolidation.    Retail routing firm's save a significant amount of money for not having to operate such a system (as well as worry about the regulatory headaches associated with running such a system).  This allows them to provide much lower commissions that the (full service) or bulge bracket brokers. The trick is real time. I like to wake up in the morning, turn on my computer and see at a glance the gain or loss data on each of my stock and bond at that moment.   Companies like Ameritrde offer them, but you have to enroll and trade stock in them. Investopedia has one and so does marketwatch    I've always used marketwatch, and I have a few current competitions going on if you want me to send the link    They recently remodeled the website so it works on mobile and not as well on desktop    Don't know anything about the investopedia one though "Brokerages offer you the convenience of buying and selling financial products. They are usually not exchanges themselves, but they can be. Typically there is an exchange and the broker sends orders to that exchange. The main benefit that brokers offer is a simpler commission structure. Not all brokers have their own liquidity, but brokers can have their own allotment of shares of a stock, for example, that they will sell you when you make an order, so that you get what you want faster. Regarding accounts at the exchanges to track actual ownership and transfer of assets, it is not safe to assume thats how that works. There are a lot of shortcomings in how the actual exchange works, since the settlement time is 1 - 3 business days, depending on the product (so upwards of 5 to 6 actual days). In a fast market, the asset can change hands many many times making the accounting completely incorrect for extended time periods. Better to not worry about that part, but if you'd like to read more about how that is regulated look up ""Failure To Deliver"" regulations on short selling to get a better understanding of market microstructure. It is a very antiquated system." Depending on your broker, you can buy these stocks directly at the most liquid local exchanges. For instance, if you are US resident and want to to buy German stocks (like RWE) you can trade these stocks over InteractiveBrokers (or other direct brokers in the US). They offer direct access to German Xetra and other local markets.