Changing the price in a limit order
This depends on the stock exchange in question. Generally if you modify an existing order [including GTC], these are internally treated as Cancel/Replace Orders. Depending on the action, you may lose the time priority position and a new position would get assigned. More here. (f) Cancel/Replace Orders. Depending on how a quote or order is   modified, the quote or order may change priority position as follows: (1) If the price is changed, the changed side loses position and is   placed in a priority position behind all orders of the same type   (i.e., customer or non-customer) at the same price. (2) If one side's quantity is changed, the unchanged side retains its   priority position. (3) If the quantity of one side is decreased, that side retains its   priority position. (4) If the quantity of one side is increased, that side loses its   priority position and is placed behind all orders of the same type at   the same price.
There could be a number of reasons: The price hit your number ($39.70) but by the time your order hit the market, the price had gone up. Perhaps the stock went up between when you placed the stop loss and when the order was executed. A trailing stop loss will ratchet up: Very simply, the trailing stop maintains a stop-loss order at a precise percentage below the market price (or above, in the case of a short position). The stop-loss order is adjusted continually based on fluctuations in the market price, always maintaining the same percentage below (or above) the market price. where A1 is the number of trades. you may have to change the number 100 to 99 depending on how the 100th trade is charged. The idea is to use the if statement to determine the price of the trades. Once you are over the threshold the price is 14*number over threshold. Firstly what are you trading that you could lose more than you put in? If you are simply trading stocks you will not lose more than you put in, unless you are trading on margin. A limit order is basically that, a limit on the maximum price you want your buy order bought at or the minimum price you want your sell order sold at. If you can't be glued to the screen all day when you place a limit order, and the market moves the opposite way, you may miss out on your order being executed. Even if you can be in front of the screen all day, you then have to decide if you want to chase the market of miss out on your purchase or sale. For example, if a stock is trading at $10.10 and you put a limit buy order to buy 1000 shares at or below $10.00 and the price keeps moving up to $10.20, then $10.30 and then $10.50, until it closes the day at $11.00. You then have the choice during the day to miss out on buying the shares or to increase your limit order in order to buy at a higher price. Sometime if the stock is not very liquid, i.e. it does not trade very often and has low volume, the price may hit $10.00 and you may only have part of your order executed, say 500 out of your 1000 shares were bought. This may mean that you may have to increase the price of your remaining order or be happy with only buying 500 shares instead of 1000. The same can happen when you are selling (but in reverse obviously). With market order, however, you are placing a buy order to buy at the next bid price in the depth or a sell order to sell at the next offer price in the depth. See the market depth table below:  Note that this price depth table is taken before market open so it seems that the stock is somewhat illiquid with a large gap between the first and second prices in the buyers (bid) prices. When the market opened this gap is closed, as WBC is a major Australian bank and is quite liquid. (the table is for demonstration purposes only). If we pretend that the market was currently open and saw the current market depth for WBC as above, you could decide to place a limit sell order to sell 1000 shares at say $29.91. You would sell 100 shares straight away but your remaining 900 sell order will remain at the top of the Sellers list. If other Buyers come in at $29.91 you may get your whole sale completed, however, if no other Buyers place orders above $29.80 and other Sellers come into the market with sell orders below $29.91, your remaining order may never be executed. If instead you placed a market sell order you would immediately sell 100 shares at $29.91 and the remaining 900 shares at $29.80. (so you would be $99 or just over 0.3% worse off than if you were able to sell the full 1000 shares at $29.91). The question is how low would you have had to lower your limit order price if the price for WBC kept on falling and you had to sell that day? There are risks with whichever type of order you use. You need to determine what the purpose of your order is. Is it to get in or out of the market as soon as possible with the possibility of giving a little bit back to the market? Or is it to get the price you want no matter how long it takes you? That is you are willing to miss out on buying the shares (can miss out on a good buy if the price keeps rising for weeks or months or even years) or you are willing to miss out on selling them right now and can wait for the price to come back up to the price you were willing to sell at (where you may miss out on selling the shares at a good price and they keep on falling and you give back all your profits and more). Just before the onset of the GFC I sold some shares (which I had bought a few years earlier at $3.40) through a market order for $5.96. It had traded just above $6 a few days earlier, but if instead of a market order I had placed a limit order to sell at $6.00 or more I would have missed out on the sale. The price never went back up  to $6 or above, and the following week it started dropping very quickly. It is now trading at about $1.30 and has never gone back above $2.00 (5.5 years later). So to me placing a limit order in this case was very risky. Many of the Bitcoin exchanges mimic stock exchanges, though they're much more rudimentary offering only simple buy/sell/cancel orders.  It's fairly normal for retail stock brokerage accounts to allow other sorts of more complex orders, where once a certain criteria is met, (the price falls below some $ threshold, or has a movement greater than some %) then your order is executed.  The space between the current buy order and the current sell order is the bid/ask spread, it's not really about timing.  Person X will buy at $100, person Y will sell at $102.  If both had a price set at $101, they would just transact.  Both parties think they can do a little bit better than the current offer.  The width of the bid/ask spread is not universal by any means.  The current highest buy order and the current lowest sell order, are both the current price.  The current quoted market price is generally the price of the last transaction, whether it's buy or sell. I don't think user4358's explanation is correct.  A trailing LIT Sell Order adjusts downwards, i.e. if you place the order with an Aux price (in TWS it's trigger price) of 105.00 and a trailing amount of 6.00 then, assuming the ask is 100.00, TWS will add the trailing amount to the ask price and if it's less than the trigger price it will adjust. So in my example, if the market (ask) goes straight up to 105.00, nothing will be adjusted, the trigger is touched and the limit order will be placed (see below). If on the the other hand the market goes down to 99.00 then trlng amt + ask is 105.00, if it goes further down to 98.00 then the trigger price will be adjusted to 104.00 (because it's less than the current trigger), and so on. For the LIT part you have either an absolute limit price you can enter, or you have an offset limit which will be subtracted from the trigger price, in which case it is adjusted as well. So back to my example, the trigger is now 104.00 and the limit offset is say 1.00, so my limit order would be placed at 103.00 if the ask ever touches 104.00, and that in turn is only visible if the bid touches 103.00 (because it's limit-if-touched). For a buy just use the same explanation with some swapped roles, the trigger price adjust upwards when the trailing amount plus bid is larger than the current trigger, and the limit offset will be added to the trigger price. Edit Also quite succinct and worth having a look at: http://www.interactivebrokers.com/en/trading/orders/trailingLimitTouched.php Guesswork, highly subjective As for why this might be good, well, you have to believe in momentum strategies, i.e. a market that goes down, will continue to go down, if you believe that and you believe in mean reversion as well, then a trailing limit order can assist you in not buying/selling impulsively, but closer to the mean.  I've never used it that way though. What I have done, even just now to get the explanation right, is to place trailing buy and sell orders simultaneously.  You will find that you can just go in with coarse estimates and because the adjustments will go towards each other, you will end up with a narrowing band of trigger prices (as opposed to trailing stop orders which will give you a widening band of trigger prices).  If you believe in overshooting and equilibria then this can be one easy way to profit from it.  I've just sold EURUSD for 1.26420 and bought it back at 1.26380 with a trailing amount of 5pips and a limit offset of 2pips within the time of writing this. I can't say I know everything about the underlying details, but from what I understand, your limit buy adds to the bid side of open orders, and one possibility is that someone placed a market order to sell when the bid price for the stock fell to $10 which was matched to your open limit order. So using your terminology, I would say the spot bid price is what fell to $10, even if for a brief moment. Whether or not it is possible for your order to be filled when the limit buy price is deeper than the current bid price is beyond me. It may have something to do with lot sizes. It depends on how you place your stop order and the type of stop orders available from your broker. If you place a stop market order and the following day the stock opens below your stop your stock will be stopped out at or around the opening price, meaning you can potentially end up with quite a large gap. If you place a stop limit order, say you place your stop at $10.00 with a limit price of $9.90, and if the price opens below $9.90, say at $9.50, your limit sell order of $9.90 will be placed onto the market but it will not be executed until the price goes back up to $9.90 or above. The third option is to place a Guaranteed Stop Loss, and as specified you are guaranteed your stop price even if the price gaps down below your stop price. You will be paying an extra fee for the Guaranteed Stop Loss Order, and they are usually mainly available with CFD Brokers (so if you are in the USA you might be out of luck). The price is moving higher so by the time you enter your order and press buy, a new buyer has already come in at that time and taken out the lowest ask price. So you end up chasing the market as the prices keep moving higher. The solution: if you really want to be sure that you buy it and don't want to keep chasing the market higher and higher, you should put in a market order instead of a limit order. With a market order you may pay a few cents higher than the last traded price but you will be sure to have your order filled. If you keep placing limit orders you may miss out altogether, especially if the price keeps moving higher and higher. In a fast moving market a market order is always best if your aim is to be certain to buy the stock. 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. There isn't a formula like that, there is only the greed of other market participants, and you can try to predict how greedy those participants will be. If someone decided to place a sell order of 100,000 shares at $5, then you can buy an additional 100,000 shares at $5. In reality, people can infer that they might be the only ones trying to sell 100,000 shares right then, and raise the price so that they make more money. They will raise their sell order to $5.01, $5.02 or as high as they want, until people stop trying to buy their shares. It is just a non-stop auction, just like on ebay. The next day the market opens trading at 10.50, You haven't specified whether you limit order for $10.10 is to buy or sell. When the trading opens next day, it follows the same process of matching the orders. So if you have put a limit order to buy at $10.10 and there is no sell order at that price, your trade will not go through. If you have placed a limit sell order at $10.10 and there is a buyer at or higher price, it would go through. The Open price is the price of the first trade of the day. try to sell if today's google stock goes above 669$ This is Relative/Pegged-to-Primary Order with a Limit Price of $669 and an offset from National Best Offer of $0.00, but it is no different than an Market Order if the market price is $669 to begin with.  do not sell if the stock keeps climbing beyond 669 unless there is a down tick of 20cents is seen This is a Trailing Stop Order with a Trailing Amount of $0.20. It sells if the market price dropped $0.20 from the peak.  The two orders are contradictory. From your comments, I think the following is what you want: Submit Trailing Stop Order when market price is above $669.  Cancel Trailing Stop Order before the end of the day and Submit Relative/Pegged-to-Primary Order to Sell. There are a couple of things you could do, but it may depend partly on the type of orders your broker has available to you. Firstly, if you are putting your limit order the night before after close of market at the top of the bids, you may be risking missing out if bid & offer prices increase by the time the market opens the next day. On the other hand, if bid & offer prices fall at the open of the next day you should get your order filled at or below your limit price. Secondly, you could be available at the market open to see if prices are going up or down and then work out the price you want to buy at then and work out the quantity you can buy at that price. I personally don't like this method because you usually get too emotional, start chasing the market if prices start rising, or start regretting buying at a price and prices fall straight afterwards. My preferred method is this third option. If your broker provides stop orders you can use these to both get into and out of the market. How they work when trying to get into the market is that once you have done your analysis and picked a price that you would want to purchase at, you put a stop buy order in. For example, the price closed at $9.90 the previous day and there has been resistance at $10.00, so you would put a stop buy trigger if the price goes over $10, say $10.01. If your stop buy order gets triggered you can have either a buy market order or a limit order above $10.01 (say $10.02). The market order would go through immediately whilst the limit order would only go through if the price continues going to $10.02 or above. The advantage of this is that you don't get emotional trying to buy your securities whilst sitting in front of the screen, you do your analysis and set your prices whilst the market is closed, you only buy when the security is rising (not falling). As your aim is to be in long term you shouldn't be concerned about buying a little bit higher than the previous days close. On the other hand if you try and buy when the price is falling you don't know when it will stop falling. It is better to buy when the price shows signs of rising rather than falling (always follow the trend). "Market orders do not get priority over limit orders.  Time is the only factor that matters in price/time order matching when the order price is the same. For example, suppose the current best available offer for AAPL is $100.01 and the best available bid is $100.00.  Now a limit buy for $100.01 and a market buy arrive at around the same instant.  The matching engine can only receive one order at a time, no matter how close together they arrive. Let's say that by chance the limit buy arrives first.  The engine will check if there's a matching sell at $100.01 and indeed there is and a trade occurs.  This all happens in an instant before the matching engine ever sees the market buy.  Then it moves on to the market buy and processes it accordingly. On the other hand, let's say that by chance the market buy arrives first.  The engine will match it with the best available sell (at $100.01) and a trade occurs.  This all happens in an instant before the matching engine ever sees the limit buy.  Then it moves on to the limit buy and processes it accordingly. So there's never a comparison between the two orders or their ""priorities"" because they never exist in the system at the same time.  The first one to arrive is processed first; the second one to arrive is processed second." Having a highly liquid emergency fund can lubricate the wheels for disaster recovery.  For example, several years ago I returned from a vacation to discover that, during my absence, a plumbing fixture had broken and my house was flooded. Since we had sufficient liquidity to cover the cost of the repairs in our emergency fund, the insurance company was much easier to deal with, and the relationships between the contractor, bank, and insurance company were much smoother. The bank was able to approve the insurance in minutes versus days. Ironically, we didn't actually have to touch our emergency fund precisely because we had it. Clarification - I make it a point to have no debt.