How do exchanges match limit orders?
The Limit Order are matched based on amount and time. The orders are listed Highest to Lowest on the Buy Side. The orders are listed Lowest to Highest on the Sell Side. If there are 2 Sell orders for same amount the order which is first in time [fractions of milliseconds] is first.  The about is the example as to how the orders would look like on any exchange.  Now the highest price the buyer is ready to pay is 20.21 and the lowest price a seller is ready to sell for is 20.25. Hence there is no trade. Now if a new Buy order comes in at 20.25, it matches with the sell and the deal is made. If a new Buy order comes in at 20.30, it still matches at 20.25. Similarly if a Sell order come in at 20.21, it matches and a deal is made. If a Sell order come in at 20.11, it still matches 20.21.   Incase of market order, with the above example if there is a Buy order, it would match with the lowest sell order at 20.25, if there is not enough quantity , it would match the remaining quantity to the next highest at 20.31 and continue down. Similarly if there is a Sell market order, the it would match to the maximum a seller is ready to buy, ie 20.21, if there is not sufficient buy quantity at 20.21, it will match with next for 20.19   If say there are new buy order at 20.22 and sell orders at 20.24, these will sit first the the above queue to be matched. In your above example the Lowest Sell order was at 20.10 at time t1 and hence any buy order after time t1 for amount 20.10 or greater would match to this and the price would be 20.10. However if the Buy order was first ie at t1 there was a buy order for 20.21 and then at time later than t1, there is a sell order for say 20.10 [amount less than or equal to 20.21] it would match for 20.21. Essentially the market looks at who was the first to sell at lower price or who was the first to buy at higher price and then decide the trade.  Edit [To Clarify xyz]: Say if there is an Sell order at $10 Qty 100. There is a buyer who is willing to pay Max $20 and is looking for Qty 500. Your key assumption that the Buyer does not know the current SELL price of $10 is incorrect. Now there are multiple things, the Buyer knows the lowest Sell order is at $10, he can put a matching Buy order at $10 Qty 100, and say $11 Qty 100 etc. This is painful. Second, lets say he puts a Buy order at $10 Qty 100, by the time the order hits the system someone else has put the trade at $10 and his order is fulfilled. So this buyer has to keep looking at booking and keep making adjustments, if its a large order, it would be extremely difficult and frustrating for this Buyer. Hence the logic of giving preference. The later Buy order says ... The Max I can pay is $20, match eveything at the current price and get the required shares. The total limit book is a composite of all the orders on all of the exchanges. While it's uncommon for a limit order posted beyond the NBBO to fill outside of the NBBO, it does occur. For example, the best ask may be on exchange X, but for some reason the smart order routing algorithm may select exchange Y if it judges the net trade to be less costly, malfunctions, etc, and HFTs will immediately arbitrage the order between two exchanges, or the best order on exchange X disappears causing the order to fill above the NBBO. The system isn't perfect because there are multiple exchanges, but that eventuality is extremely rare with equities since nearly every exchange will have orders posted at the NBBO because exchange equity fee and rebate schedules are extremely competitive, nearly identical.  It is however more common with options since less exchanges as a percentage of the total will have orders posted at the NBBO because of very wide exchange rebate and fee schedules. How a single exchange handles a new order that crosses an existing limit order is already addressed here: How do exchanges match limit orders?
In practice, it would not work. If you put a bid in that was really out of line, even if it got filled, the exchange would reverse it.  Other than that it really depends on what the current bid/ask spread it, and what volume its trading, as well as how the market feels. Say the current bid is 11 and you put an order in with bid 11.5, it would soak up all the orders on the market up to the volume your buying. But once your order is filled the market will be determined by what the next order's bid/ask is. It might stay where you moved it too if others feels thats a fair price. But if every other order on the market is still at 11, then the price isnt gonna move.  tl;dl unless your a market maker, you could not realistically affect the price. Depending on your strategy, it could be though there is also something to be said for what kind of order are you placing: Market, limit or otherwise? Something else to consider is whether or not there is some major news that could cause the stock/ETF to gap at the open.  For example, if a company announces strong earnings then it is possible for the stock to open higher than it did the previous day and so a market order may not to take into account that the stock may jump a bit compared to the previous close.  Limit orders can be useful to put a cap on how much you'll pay for a stock though the key would be to factor this into your strategy of when do you buy. I assume you're talking about a sell order, not a buy order. When you place a limit sell order, your order is guaranteed to be placed at that price or higher.  If the market is currently trading much higher than the price of your sell order, then your mistakenly low limit order will be essentially a market order, and will be filled at  the current bid price. So the only way this is a problem is if you want to place a limit sell that is much higher than the current market, but mistakenly place a limit lower than the current market. "After learning about things that happened in the ""flash crash"" I always use limit orders. In an extremely rare instance if you place a market order when there is a some glitch, for example some large trader adds a zero at the end of their volume, you could get an awful price.  If I want to buy at the market price, I just set the limit about 1% above the market price. If I want to sell, I set the limit 1% below the market price. I should point out that your trade is not executed at the limit price. If your limit price on a buy order is higher than the lowest offer, you still get filled at the lowest offer. If before your order is submitted someone fills all offers up to your limit price, you will get your limit price. If someone, perhaps by accident, fills all orders up to twice your limit price, you won't end up making the purchase. I have executed many purchases this way and never been filled at my limit price." "This is a great question precisely because the answer is so complicated. It means you're starting to think in detail about how orders actually get filled / executed rather than looking at stock prices as a mythical ""the market"". ""The market price"" is a somewhat deceptive term. The price at which bids and asks last crossed & filled is the price that prints. I.e. that is what you see on a market price data feed. ] In reality there is a resting queue of orders at various bids & asks on various exchanges.  (source: Larry Harris. A size of 1 is 1H = 100 shares.) So at first your 1000H order will sweep through the standing queue of fills. Let's say you are trading a low-volume stock. And let's say someone from another brokerage has set a limit order at a ridiculous price. Part of your order may sweep through and part of it get filled at a ridiculously high price. Or maybe either the exchange or your broker / execution mechanism somehow will protect you against the really high fill. (Let's say your broker hired GETCO, who guarantees a certain VWAP.) Also people change their bids & asks in response to what they see others do. Your 1000H size will likely be marked as a human counterparty by certain players. Other players might see that order differently. (Let's say it was a 100 000H size. Maybe people will decide you must know something and decide they want to go the same direction as you rather than take the opportunity to exit. And maybe some super-fast players will weave in and out of the filling process itself.) There is more to it because, what if some of the resting asks are on other venues? What if both you and some of the asks match with someone who uses the same broker as you? Not only do exchange rules come into play, but so do national regulations. tl;dr: You will get filled, with price slippage. If you send in a big buy order, it will sweep through the resting asks but also there are complications." "There needs to be a buyer of the shares you are offering.  There are a lot of feature rich options for buying and selling.  I don't understand them all in depth, but for example on TD Ameritrade here are some of the order types ""Limit"", ""Market"", ""Stop Market"", ""Stop Limit"", ""Trailing Stop %"", ""Trailing Stop $"".  This web page will explain the different order types https://invest.ameritrade.com/cgi-bin/apps/u/PLoad?pagename=tutorial/orderTypes/overview.html Stock with a higher volume will allow your trade to execute faster, since there are more frequent trades than stocks with lower volume. (UPDATE: More specifically, not more frequent trades, but more shares changing hands.) I'm a bit of a noob myself, but that's what I understand." To answer your question in its entirety there's more information we need (exchange, session, traded security, order type, etc.). Most exchanges support partial fills, that is your order will be partially executed and modified.  In your example, you'd get an execution of 10 shares at $100, and your order ticket will be modified to $100 for 990 shares. Like John Bensin explained, there are ways to prevent partial filling through order modifiers (e.g. Fill-or-Kill). My addition here is, there are also ways to prevent the other bit, i.e. do the partial fill but don't keep a modified order in the system.  You'd have to mark the order Immediate-or-Cancel (IoC).  In your case you'd be partially filled (10 @$100) and that's it.  For the remaining 990 shares you'd have to enter a new order. Probing for hidden limit orders usually involves sending the orders and then cancelling them before they get filled if they don't get filled. With trades actually going through multiple times for small amounts it looks more like a VWAP strategy where the trader is feeding small volumes into the market as part of a larger trade trying to minimize average cost. It could be probing but without seeing the orders and any cancels it would be difficult to tell. edit: I just had another thought; it could possibly be a market maker unwinding a bad position caused by other trading. Sometimes they drip trades into the market to prevent themselves from hitting big orders etc. that might move back against them. This is probably not right but is just another thought. source: I work for an organization that provides monitoring for these things to many large trading organizations. A limit order is simply an order to buy at a maximum price or sell at a minimum price. For example, if the price is $100 and you want to sell if the price rises to $110, then you can simply put a limit order to sell at $110. The order will be placed in the market and when the price reaches $110 your order will be executed. If the price gaps at the open to $111, then you would end up selling for $111. In other words you will get a minimum of $110 per share. A stop limit order is where you put a stop loss order, which when it gets triggered, will place a limit order in the market for you. For example, you want to limit your losses by placing a stop loss order if the price drops to $90. If you chose a market order with your stop loss as soon as the price hits $90 your stop loss would be triggered and the shares would sell at the next available price, usually at $90, but could be less if the market gaps down past $90. If on the other hand you placed a limit order at $89.50 with your stop loss, when the stop loss order gets triggered at $90 your limit order will be placed into the market to sell at $89.50. So you would get a minimum of $89.50 per share, however, if the market gaps down below $89.50 your order will be placed onto the market but it won't sell, unless the price goes back to or above $89.50. Hope this helps. "Just read the book Flash Boys by Michael Lewis.  It describes this process in detail, albeit a bit more dramatically than it has to.  Basically what ""HFTs"" (high frequency traders) do is they set up their line to the exchange so its microseconds faster than everyone else.  Then they test out the market with tiny orders, seeing how fast it's getting filled - if these are getting filled immediately, it probably means there's a big order coming in from an investor.  So the algorithm - and it's all algo-based obviously because no human can remotely hope to catch this - will detect that as soon as there's a spike in volume, it will buy all of the volume at the current price, and sell it back for higher, forcing the big investor to take on higher prices.    Another case is that some HFTs can basically buy the entire trade book from an exchange like Nasdaq.  So every time someone places a market order for 200 shares of a 6.5 stock, the HFT will see it, buy up all the current stock from say 6.5-6.6 and sell it back at 6.7.  Not rocket science if you already get info about the trade coming in, in fact this is basically market making but performed by an ""evil HFT"" instead of a ""trustworthy bank.""    But honestly there are a lot more ways to make money from HFT than front-running, which isn't even possible anymore because exchanges no longer sell their books to HFTs." Yes for every order there is a buyer and seller. But overall there are multiple buyers and multiple sellers. So every trade is at a different price and this price is agreed by both buyer and seller. Related question will help you understand this better. How do exchanges match limit orders? Different exchanges sometimes offer different order types, and of course have different trading fees.  But once a trade is finished, it should not matter where it was executed. A few observations - A limit order can certainly work, as you've seen.  I've put in such an order far beyond the true value, and gotten back a realistic bid/ask within 10 minutes or so. That at least gave me an idea where to set my limit.  When this doesn't work, an exercise is always another way to go. You'll get the full intrinsic value, but no time value, by definition.  Per your request in comment - You own a put, strike price $100. The stock (or ETF) is trading at $50. You buy the stock and tell the broker to exercise the put, i.e. deliver the stock to the buyer of the put. You're losing money. And a lot of it. Consider this: the inflation is 2-4% a year (officially, depending on your spending pattern your own rate might be quite higher). You earn about 1/2%. I.e.: You're losing 3% a year. Guaranteed. You can do much better without any additional risk. 0.1% on savings account? Why not 0.9%? On-line savings account (Ally, CapitalOne-360, American Express, E*Trade, etc) give much higher rates than what you have. Current Ally rates are 0.9% on a regular savings account. 9 times more than what you have, with no additional risk: its a FDIC insured deposit. You can get a slightly higher rate with CDs (0.97% at the same bank for 12 months deposit). IRA - why is it in CD's? Its the longest term investment you have, that's where you can and should take risks, to maximize your compounding returns. Not doing that is actually more risky to you because you're guaranteeing compounding loss, of the said 3% a year. On average, more volatile stock investments have shown to be not losing money over periods of decades, even if they do lose money over shorter periods. Rental - if you can buy a property that you would pay the same amount of money for as for a comparable rental - you should definitely buy. Your debt will be secured by the property, and since you're paying the same amount or less - you're earning the equity. There's no risk here, just benefits, which again you chose to forgo. In the worst case if you default and walk away from the property you lost exactly (or less) what you would have paid for a rental anyway. 14 years old car may be cheaper than 4 years old to buy, but consider the maintenance, licensing and repairs - will it not some up to more than the difference? In my experience - it is likely to. Bottom line - you think you're risk averse, but you're exactly the opposite of that. Ehm, the 2008 crisis was effectively a debt crisis. Debt levels are  higher now than in 2008 and default rates are on the rise. Furthermore the ability for monetary policy to absorb has been spent. What helped in 2008 was the high savings rate of China. Now even china is in a debt bubble.  Stocks are beyond expensive in a historical context, and the reason is, with money so cheap where else do you stand to get some return? This crash is coming... not sure when as market timing is impossible, but i would advise you to take a big crisis into account in your planning...,