What happens when the bid and ask are the same?
This question is impossible answer for all markets but there are 2 more possibilities in my experience: 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. In simple terms, this is how the shares are traded, however most of the times market orders are placed. Consider below scenario( hypothetical scenario, there are just 2 traders) Buyer is ready to buy 10 shares @ 5$ and seller is ready to sell 10 shares @ 5.10$, both the orders will remain in open state, unless one wish to change his price, this is an example of limit order. Market orders If seller is ready to sell 10 shares @ 5$ and another 10 shares @5.05$, if buyer wants to buy 20 shares @ market price, then the trade will be executed for 10 shares @ 5$ and another 10 shares @ 5.05$ Rule 610 (Google for it) stands that if Bid and Ask are the same, the market is considered Locked, and the exchange must stop all trading. So the same person can't quote the same bid and ask price. However, HFTs have found ways to circumvent this limitation when exchanges created special order types for them, e.g. Spam-and-Cancel
"It's good to ask this question, because this is one of the fundamental dichotomies in market microstructure. At any time T for each product on a (typical) exchange there are two well-defined prices: At time T there is literally no person in the market who wants to sell below the ask, so all the people who are waiting to buy at the bid (or below) could very well be waiting there forever.  There's simply no guarantee that any seller will ever want to part with their product for a lesser price than they think it's worth. So if you want to buy the product at time T you have a tough choice to make: you get in line at the bid price, where there's no guarantee that your request will ever be filled, and you might never get your hands on the product you decide that owning the product right now is more valuable to you than (ask - bid) * quantity, so you tell the exchange that you're willing to buy at the ask price, and the exchange matches you with whichever seller is first in line Now, if you're in the market for the long term, the above choice is completely immaterial to you.  Who cares if you pay $10.00 * 1000 shares or $10.01 * 1000 shares when you plan to sell 30 years from now at $200 (or $200.01)? But if you're a day trader or anyone else with a very short time horizon, then this choice is extremely important: if the price is about to go up several cents and you got in line at the bid (and never got filled) then you missed out on some profit if you ""cross the spread"" to buy at the ask and then the price doesn't go up (or worse, goes down), you're screwed.  In order to get out of the position you'll have to cross the spread again and sell at at most the bid, meaning you've now paid the spread twice (plus transaction fees and regulatory fees) for nothing. (All of the above also applies in reverse for selling at the ask versus selling at the bid, but most people like to learn in terms of buying rather than selling.)" But what about the following scenario which is my paraphrasing of a Nanex article (I'm hoping you can help clarify this for me).  1. I observe a 1,000 lot @$10 advertised for Sell on a lit exchange.  2. I try to lift the 1,000 by placing a limit order @10.  3. My order goes through some kind of order routing process. First, 3 orders get executed on a dark pool. Let's say I got a 50 lot filled (so available offer reduces to 950).  4. My order hits a lit exchange. I get a partial fill for 100 (offer shrinks to 850); but the offered size shrinks instead to 500. Or 0.  5. Now, in order to execute my trade, I will have to take a higher price than the original advertised liquidity.  My question (maybe you can answer this) is why did my original order size of 1,000 appear in smaller blocks? Is this because the order routing algorithm breaks up the size?  Or is it that market makers only post offers in small block sizes (e.g. 100) So even if the order book looked like:  100 @ 10  100 @ 10  100 @ 10  100 @ 10  100 @ 10  100 @ 10  All the way to 1,000 total -- as soon as the first 100 shares were lifted, the MM can immediately cancel the remainder of the advertised liquidity -- in practical terms making it impossible to execute large orders at an advertised price. When there is a difference between the two ... no trading occurs. Let's look at an example: Investor A, B, C, and D all buy/sell shares of company X. Investor A wants to sell 10 shares at $20 a share (Ask price $20 x10). Investor B wants to buy 15 shares at $10 a share (Bid price $10 x15). Since the bid price and ask price are different, no sale is made. Next Investor C comes along and wants to sell 5 shares at $14 (Ask price $14 x5). Still no sale. Investor D comes along and wants to buy 5 shares for $14 each. So a sale is finally made. At this point, the stock quote moves to $14. The ask price is $20 x10 and the bid price is $10 x15. No further trading will occur until another investor is willing to buy at $20 or sell at $10.  Another discussion of this topic is shown on this post. This sometimes happens to me. It depends on how liquid the option is. Normally what I see happening is that the order book mutates itself around my order. I interpret this to mean that the order book is primarily market makers. They see a retail investor (me) come in and, since they don't have any interest in this illiquid option, they back off. Some other retail investor (or whatever) steps in with a market order, and we get matched up. I get a fill because I become the market maker for a brief while. On highly liquid options, buy limits at the bid tend to get swallowed because the market makers are working the spread. With very small orders (a contract or two) on very liquid options, I've had luck getting quick fills in the middle of the spread, which I attribute to MM's rebalancing their holdings on the cheap, although sometimes I like to think there's some other anal-retentive like me out there that hates to see such a lopsided book. :) I haven't noticed any particular tendency for this to happen more with puts or calls, or with buy vs sell transactions. For a while I had a suspicion that this was happening with strikes where IV didn't match IV of other strikes, but I never cared enough to chase it down as it was a minor part of my overall P/L. "In the first situation you describe, any intelligent routing algo will send a 1000 lot order to the lit exchange in step 1. Then you get filled 1000@$10. After the fill occurs, the matching engine tells everyone what happened.   If the order book consists of 100 orders of 1 lot @ $10, and you place a ""buy 100 lots"" order, here is what happens:  1. The matching engine receives your order. 2. The matching engine matches your order against the 100 individual orders on the book. 3. The matching engine broadcasts 100 trade notifications.   No one has any opportunity to cancel their orders since they only hear of the fill after it happened.   The only way someone would have the opportunity to cancel is if there was 500 lots on one exchange and 500 on another. Then someone might observe a trade on exchange #1 and cancel their sell order on exchange #2 in response." One broker told me that I have to simply read the ask size and the bid size, seeing what the market makers are offering. This implies that my order would have to match that price exactly, which is unfortunate because options contract spreads can be WIDE. Also, if my planned position size is larger than the best bid/best ask, then I should break up the order, which is also unfortunate because most brokers charge a lot for options orders. The order book looks fine, if it were a liquid market. However, a bid that matches with an ask will always be met on a first come first serve basis. There's no other way to do it.   Most traders don't like doing that because they want to try to get a lower price. HFT don't have to worry about meeting the ask because they're just going to pass that cost on to the guy on the hook.   By the time the HFT makes the buy they already know the guy wants to buy at 70.00  They did not know that at the time they placed the buy order. If the buy order from the HFT hasn't been cancelled it means they already found someone.  How? By testing the market with sell orders at the same time they were sending buy orders.  They keep a little bit of stock in reserve to perform these tests. "I place a trade, a limit order on a thinly traded stock. I want to buy 1000 shares at $10. The current price is $10.50. Someone places a market order for 500 shares. Another trader has a limit order for $10.10 for 400 shares. His order fills, and I get 100 at my price. I wait another day to see if I get any more shares. This is just an example of how it can work. I can place my order as ""all or none"" if I wish to avoid this." "The current stock price you're referring to is actually the price of the last trade.  It is a historical price – but during market hours, that's usually mere seconds ago for very liquid stocks. Whereas, the bid and ask are the best potential prices that buyers and sellers are willing to transact at: the bid for the buying side, and the ask for the selling side.  But, think of the bid and ask prices you see as ""tip of the iceberg"" prices.  That is: The ""Bid: 13.20 x200"" is an indication that there are potential buyers bidding $13.20 for up to 200 shares.  Their bids are the highest currently bid; and there are others in line behind with lower bid prices.  So the ""bid"" you're seeing is actually the best bid price at that moment.  If you entered a ""market"" order to sell more than 200 shares, part of your order would likely be filled at a lower price. The ""Ask: 13.27 x1,000"" is an indication that there are potential sellers asking $13.27 for up to 1000 shares.  Their ask prices are the lowest currently asked; and there are others in line behind with higher ask prices.  So the ""ask"" you're seeing is the best asking price at that moment.  If you entered a ""market"" order to buy more than 1000 shares, part of your order would likely be filled at a higher price. A transaction takes place when either a potential buyer is willing to pay the asking price, or a potential seller is willing to accept the bid price, or else they meet in the middle if both buyers and sellers change their orders. Note: There are primarily two kinds of stock exchanges.  The one I just described is a typical order-driven matched bargain market, and perhaps the kind you're referring to. The other kind is a quote-driven over-the-counter market where there is a market-maker, as JohnFx already mentioned.  In those cases, the spread between the bid & ask goes to the market maker as compensation for making a market in a stock.  For a liquid stock that is easy for the market maker to turn around and buy/sell to somebody else, the spread is small (narrow).  For illiquid stocks that are harder to deal in, the spread is larger (wide) to compensate the market-maker having to potentially carry the stock in inventory for some period of time, during which there's a risk to him if it moves in the wrong direction.    Finally ... if you wanted to buy 1000 shares, you could enter a market order, in which case as described above you'll pay $13.27.  If you wanted to buy your shares at no more than $13.22 instead, i.e. the so-called ""current"" price, then you would enter a limit order for 1000 shares at $13.22.  And more to the point, your order would become the new highest-bid price (until somebody else accepts your bid for their shares.)  Of course, there's no guarantee that with a limit order that you will get filled; your order could expire at the end of the day if nobody accepts your bid." "If there is a very sudden and large collapse in the exchange rate then because algorithmic trades will operate very fast it is possible to determine “x” immediately after the change in exchange rate. All you need to know is the order book. You also need to assume that the algorithmic bot operates faster than all other market participants so that the order book doesn’t change except for those trades executed by the bot. The temporarily cheaper price in the weakened currency market will rise and the temporarily dearer price in the strengthened currency market will fall until the prices are related by the new exchange rate. This price is determined by the condition that the total volume of buys in the cheaper market is equal to the total volume of sells in the dearer market.   Suppose initially gold is worth $1200 on NYSE or £720 on LSE. Then suppose the exchange rate falls from r=0.6 £/$ to s=0.4 £/$.  To illustrate the answer lets assume that before the currency collapse the order book for gold on the LSE and NYSE looks like: GOLD-NYSE   Sell (100 @ $1310) Sell (100 @ $1300) <——— Sell (100 @ $1280) Sell (200 @ $1260) Sell (300 @ $1220) Sell (100 @ $1200) ————————— buy (100 @ $1190) buy (100 @ $1180)   GOLD-LSE Sell (100 @ £750) Sell (100 @ £740) ————————— buy (200 @ £720) buy (200 @ £700) buy (100 @ £600) buy (100 @ £550) buy (100 @ £530) buy (100 @ £520) <——— buy (100 @ £500)   From this hypothetical example, the automatic traders will buy up the NYSE gold and sell the LSE gold in equal volume until the price ratio ""s"" is attained. By summing up the sell volumes on the NYSE and the buy volumes on the LSE, we see that the conditions are met when the price is $1300 and £520. Note 800 units were bought and sold.   So “x” depends on the available orders in the order book. Immediately after this, however, the price of the asset will be subject to the new changes of preference by the market participants. However, the price calculated above must be the initial price, since otherwise an arbitrage opportunity would exist." It this a real situation or is it a made up example? Because for a stock that has a last traded priced of $5 or $6 and volume traded over $4M (i.e. it seems to be quite liquid), it is hardly likely that the difference from bid to ask would be as large as $1 (maybe for a stock that has volume of 4 to 5 thousand, but not for one having volume of 4 to 5 million). In regards to your question, if you were short selling the order would go in exactly the same as if you were selling a stock you owned. So your order would be on the ask side and would need to be matched up with a price on the bid side for there to be a trade. "The market price of a stock is based on nothing at all more than what two parties were last willing to transact for it. The stock has a ""bid"" and an ""ask"" each is the value placed by a counterparty. For the sale to occur, one party must meet the other. The stock transacts and that is the price. For a stock to ""go up"" people must be willing to pay more for it. Likewise, for it to ""go down"" people must be willing to accept less for it." The obvious thing would happen. 10 shares change owner at the price of $100. A partially still open selling order would remain. Market orders without limits means to buy or sell at the best possible or current price. However, this is not very realistic. Usually there is a spread between the bid and the ask price and the reason is that market makers are acting in between. They would immediately exploit this situation, for example, by placing appropriately limited orders. Orders without limits are not advisable for stocks with low trading activity. Would you buy or sell stuff without caring for the price? "Market makers (shortened MM) in an exchange are generally required to list both a bid and ask price to allow both buyers and sellers to trade and keep the market moving.  However, a more general idea of a MM may includes companies off an exchange (say large banks acting as broker/dealers in an over-the-counter market) are not required to give a simultaneous bid/ask, but often will on request.  So, it might depend on where you are getting this data but likely the bid/ask was quoted simultaneously. An exchange, like the NASDAQ for instance, may have multiple MMs for a given market.  The ""market"" spread will be from the highest bid to the lowest ask over all the MMs.  The highest bid and lowest ask may come from different MMs and any particular MM often will have a larger spread. The size of the spread gives a rough idea of how much a MM is trying to make off of a ""round trip"" trade (buying than immediately selling to someone else or selling than immediately buying from someone else).  Of course, immediate round-trip trades are not always possible and there are many other complications.  However, half the spread is a rough indicator of how much they hope to make off of a single trade." 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?