What will happen when a bid price is higher than an ask price?
It depends on the sequence in which the order [bid and ask] were placed. Please read the below question to understand how the order are matched. How do exchanges match limit orders? The situation you're proposing is an over-simplification that wouldn't occur in practice. Orders occur in a sequence over time. Time is an important part of the order matching process. Orders are not processed in parallel; otherwise, the problem of fairness, already heavily regulated, would become even more complex. First, crossed and locked markets are forbidden by regulators.  Crossed orders are where one exchange has a higher bid than another's ask, or a lower ask than another's bid.  A locked market is where a bid on one exchange is equal to the ask on another.  HFTs would be able to make these markets because of the gap between exchange fees. Since these are forbidden, and handling orders in parallel would ensure that a crossed or locked market would occur, orders are serialized (queued up), processed in order of price-time priority. So, the first to cross the market will be filled with the best oldest opposing order. Regulators believe crossed or locked markets are unfair.  They would however eliminate the bid ask spread for many large securities thus the bid-ask cost to the holder.
It definitely depends on the exchange you are trading on. I'm not familiar with Scottrade, but a standard practice is to fulfill limit orders in the order they are placed.   Most of the time, you wouldn't see stocks trade significantly under your bid price, but since penny stocks are very volatile, it's more likely their price could drop quickly past your bid and then return above it while only fulfilling a portion of the orders placed. Example 1. Penny stock priced at $0.12 2. Others place limit orders to buy at $0.10 3. You place limit order to buy at $0.10 4. Stock price drops to $0.07 and some orders are filled (anything $0.07 or higher) based on a first-come first-served basis 5. Due to the increase in purchases of the penny stock, the price rises above $0.10 before your order is filled ***EDIT*** - Adding additional clarification from comment section. A second example If the price drops from $0.12 to $0.07, then orders for all prices from $0.07 and above will start to be filled from the oldest order first. That might mean that the oldest order was a limit buy order for 100 shares at $0.09, and since that is above the current ask price, it will be filled first. The next order might be for 800 shares at $0.07. It's possible for a subset of these to be filled (let's say 400) before the share's price increases from the increased demand. Then, if the price goes above $0.10, your bid will not be filled during that time. In the world of stock exchanges, the result depends on the market state of the traded stock. There are two possibilities, (a) a trade occurs or (b) no trade occurs. During the so-called auction phase, bid and ask prices may overlap, actually they usually do. During an open market, when bid and ask match, trades occur. If you are buying your order will be placed in Bid list. If you are selling your order will be placed in the Ask list. The highest Bid price will be placed at the top of the Bid list and the lowest Ask price will be placed at the top of the Ask list. When a Bid and Ask price are matched a transaction will take place and it will the last traded price. If you are looking to buy at a lower price, say $155.01, your Bid price will be placed 3rd in the Bid list, and unless the Ask prices fall to that level, your order will remain in the list until it trades, it expires or you cancel it. If prices don't fall to you Bid price you will not get a trade. If you wanted your trade to go through you could either place a limit buy order closer to the lowest Ask price (however this is still not a certainty), or to be certain place a market buy order which will trade at the lowest Ask price. The principle of demand-supply law will not work if spoofing (or layering, fake order) is implemented. However, spoofing stocks is an illegal criminal practice  monitored by SEC. In stock market, aggressive buyer are willing to pay for a higher ask price pushing the price higher even if ask size is considerably larger than bid size, especially when high growth potential with time is expected. Larger bids may attract more buyers, further perpetuating a price increase (positive pile-on effect). Aggressive sellers are willing to accept a lower bid price pushing the price lower even if ask size is considerably smaller than bid size, when a negative situation is expected. Larger asks may attract more sellers, further perpetuating a price fall (negative pile-on effect).  Moreover, seller and buyers considers not only price but also size of shares in their decision-making process, along with marker order and/or limit order. Unlike limit order, market order is not recorded in bid/ask size. Market order, but not limit order, immediately affects the price direction. Thus, ask/bid sizes alone do not give enough information on price direction. If stocks are being sold continuously at the bid price, this could be the beginning of a downward trend; if stocks are being sold continuously at the ask price, this could be the beginning of a upward trend.  This is because ask price is always higher than bid price. In all the cases, both buyers and sellers hope to make a profit in a long-term and short-term view "Buying pressure is when there are more buy orders than sell orders outstanding.  Just because someone wants to buy a stock doesn't mean there's a seller ready to fill that order.  When there's buying pressure, stock prices rise.  When there's selling pressure, stock prices fall. There can be high volume where buying and selling are roughly equal, in which case share prices wouldn't move much. The market makers who actually fill buy and sell orders for stock will raise share prices in the face of buying pressure and lower them in the face of selling pressure.  That's because they get to keep the margin between what they bought shares from a seller for and what they can sell them to a new buyer for.  Here's an explanation from InvestorPlace.com about ""buying pressure"": Buying pressure can basically be defined as increasingly higher demand for a particular stock's shares. This demand for shares exceeds the supply and causes the price to rise. ... The strength or weakness of a stock determines how much buying or selling interest will be required to break support and resistance areas. I hope this helps!" "Stock prices are set by bidding. In principle, a seller will say, ""I want $80."" If he can't find anyone willing to buy at that price, he'll either decide not to sell after all, or he'll lower his price. Likewise, a buyer will say, ""I'll pay $70."" If he can't find anyone willing to pay that price, he'll either decide not to buy or he'll increase his price. For most stocks there are many buyers and many sellers all the time, so there's a constant interplay. The typical small investor has VERY little control of the price. You say, ""I want to buy 10 shares of XYZ Corporation and my maximum price is $20."" If the current trending price is below $20, your broker will buy it for you. If not, he won't. You normally have some time limit on the order, so if the price falls within your range within that time period, your broker will buy. That is, your choice is basically to buy or not buy, or sell or not sell, at the current price. You have little opportunity to really negotiate a better price. If you have a significant percentage of a company's total stock, different story. In real life, most stocks are being traded constantly, so buyers and sellers both have a pretty good idea of the current price. If the last sale was ten minutes ago for $20, it's unlikely anyone's going to now bid $100. They're going to bid $20.50 or $19.25 or some such. If the last sale was for $20 and your broker really came to the floor and offered to buy for $100, I suppose someone would sell to him very quickly before he realized what an outrageous price this was. I use TD Ameritrade, and on their web site, if I give a price limit on a buy that's more than a small percentage above the last sale, they reject it as an error. I forget the exact number but they won't even accept a bid of $80 if the stock is going for $40. They might accept $41 or $42, something like that." "Your scenario depicts 2 ""in the money"" options, not ""at the money"". The former is when the share price is higher than the option strike, the second is when share price is right at strike. I agree this is a highly unlikely scenario, because everyone pricing options knows what everyone else in that stock is doing. Much about an option has everything to do with the remaining time to expiration. Depending on how much more the buyer believes the stock will go up before hitting the expiration date, that could make a big difference in which option they would buy. I agree with the others that if you're seeing this as ""real world"" then there must be something going on behind the scenes that someone else knows and you don't. I would tread with caution in such a situation and do my homework before making any move.   The other big factor that makes your question harder to answer more concisely is that you didn't tell us what the expiration dates on the options are. This makes a difference in how you evaluate them. We could probably be much more helpful to you if you could give us that information." 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. "Number 2 cannot occur. You can buy the call back and sell the stock, but the broker won't force that #2 choice. To trade options, you must have a margin account. No matter how high the stock goes, once ""in the money"" the option isn't going to rise faster, so your margin % is not an issue.  And your example is a bit troublesome to me. Why would a $120 strike call spike to $22 with only a month left? You've made the full $20 on the stock rise and given up any gain after that. That's all. The call owner may exercise at any time. Edit: @jaydles is right, there are circumstances where an option price can increase faster than the stock price. Options pricing generally follows the Black-Scholes model. Since the OP gave us the current stock price, option strike price, and time to expiration, and we know the risk free rate is <1%, you can use the calculator to change volatility. The number two scenario won't occur, however, because a covered call has no risk to the broker, they won't force you to buy the option back, and the option buyer has no motive to exercise it as the entire option value is time premium." "But what happen if the stock price went high and then go down near expiry date? When you hold a short (sold) call option position that has an underlying price that is increasing, what will happen (in general) is that your net margin requirements will increase day by day.  Thus, you will be required to put up more money as margin to finance your position.  Margin money is simply a ""good faith"" deposit held by your broker.  It is not money that is debited as cash from the accounting ledger of your trading account, but is held by your broker to cover any potential losses that may arise when you finally settle you position.   Conversely, when the underlying share price is decreasing, the net margin requirements will tend to decrease day by day.   (Net margin is the net of ""Initial Margin"" and ""Variation Margin"".) As the expiry date approaches, the ""time value"" component of the option price will be decreasing." 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 think that if the price does not go very far up, then your order will open on 101, because you are setting a limit order, if suddenly the price goes up very quickly or with a gep even, then you may not be given a position. But this is with a limit order and it is better to check with the broker. There are also warrants in which you can adjust the price range, for example, from 101 to 103, and at a sharp price jump, it is possible for you and would not give a position at a price of 101, but perhaps 103 would get. "You might consider learning how the ""matching"" or ""pairing"" system in the market operates.  The actual exchange only happens when both a buyer and a seller overlap their respect quotes.   Sometimes orders ""go to market"" for a particular volume.  Eg get me 10,000 Microsoft shares now.  which means that the price starts at the current lowest seller, and works up the price list until the volume is met.  Like all market it trades, it has it's advantages, and it's dangers.  If you are confident Microsoft is going to bull, you want those shares now, confident you'll recoup the cost.  Where if you put in a priced order, you might get only none or some shares. Same as when you sell.  If you see the price (which is the price of the last completed ""successful"" trade.  and think ""I'm going to sell 1000 shares"". then you give the order to the market (or broker), and then the same as what happened as before.  the highest bidder gets as much as they asked for, if there's still shares left over, they go to the next bidder, and so on down the price... and the last completed ""successful"" trade is when your last sale is made at the lowest price of your batch. If you're selling, and selling 100,000 shares. And the highest bidder wants 1,000,000 shares you'll only see the price drop to that guys bid.  Why will it drop (off the quoted price?).  Because the quoted price is the LAST sale, clearly if there's someone still with an open bid on the market...then either he wants more shares than were available (the price stays same), or his bid wasn't as high as the last bid (so when you sale goes through, it will be at the price he's offering).   Which is why being able to see the price queues is important on large traders. It is also why it can be important put stops and limits on your trades, een through you can still get gapped if you're unlucky.  However putting prices (""Open Orders"" vs ""(at)Market Orders"") can mean that you're sitting there waiting for a bounce/spike while the action is all going on without you). safer but not as much gain (maybe ;) ) that's the excitement of the market, for every option there's advantages...and risks... (eg missing out) There are also issues with stock movement, shadowing, and stop hunting, which can influence the price. But the stuff in the long paragraphs is the technical reasons." The market maker will always compare the highest bid and the lowest ask. A trade will happen if the highest bid is at least as high as the lowest ask. Adding one share (or a million shares) at a higher asking price, here: $210 instead of $200, will not have any effect at all. Nobody will buy the share.  Adding a bid for one share (or a million shares) at a higher bid price will trigger a sale. If you bid $210 for one share, you will pay $210 for one of the shares that were offered at $200. If you have $210 million in cash and add a bid for 1,000,000 AAPL at $210, you will pay $210 for all shares with an ask of $200.00, then $200.01, then $200.02 until you either bought all shares with an ask up to $210, or until you bought a million shares.  With AAPL, you probably bid the price up to $201 with a million shares, so you made lots of people very happy while losing about 10 million dollars. So let's say this is a much smaller company.  You have driven the share price up to $210, but there is nobody else bidding above $200. So nobody is going to buy your shares. Until some people think there is something going on and enter higher bids, but then some people will take advantage of this and ask lower than your $210. And there will be more people trying to make cash by selling their shares at a good price than people tricked into bidding over $200, so it is most likely that you lose out.  (This completely ignores legality; attempting to do this would be market manipulation and in many countries illegal. I don't know if losing money in the process would protect you from criminal charges). Your order may or may not be executed.  The price of stock can open anywhere.  Often yesterday's close is a good indication of today's open, but with a big event overnight, the open may be somewhere quite different.  You'll have to wait and see like the rest of us.  Also, even if it doesn't execute at the open, the price could vary during the day and it might execute later.