Is the best ask price the ask at the “top” of the order book? What is the “top” of the book?
The best ask is the lowest ask, and the best bid is the highest bid.  If the ask was lower than the bid then they crossed, and that would be a crossed market and quickly resolved. So the bid will almost always be cheaper than the ask. A heuristic is that a bid is the revenue of the stock at any given time while the ask is the cost, so the market will only ever offer a profit to itself not to the liquidity seeker. If examining the book vertically, all orders are usually sorted descending. Since the best ask is the lowest ask, it is on the bottom of the asks, and vice versa for the best bid. The best bid & best ask will be those closest since that's the narrowest spread and price-time priority will promise that a bid that crosses the asks will hit the lowest ask, the best possible price for the bidder and vice versa for an ask that crosses the best bid. "You're confusing a specific visual representation of the top bid & ask orders selected from the order book with the actual ""top of the book"".  ""Top"" in the sense of the ""top of the book"" is a ranking (by order of ""best"", different for bids vs. asks) and not meant to be strictly a visual positioning on a page or screen. The data in the visual representation comes from the top of the order book (the best bids, and the best asks), but that visual representation is choosing to present it in a specific way. Think of the ""book"" as the model, the abstract collection of outstanding bid and ask order data.  When people talk about the ""top of the book"", they're talking about the best bids (higher being better), and the best asks (lower being better). The visual representation above is but one possible way to render a tip-of-the-iceberg view of the best orders in the ""book"".  The advantage of that particular visual representation is that one can see the asks & bids converging towards the center.  The spread is visible as the difference between the two middle elements – being the lowest row in the blue ""Asks"" area, and the highest row in the green ""Bids"" area. The up-arrow they had included in the ""Asks"" area was perhaps meant to provide a clue about how the data was sorted contrary to expectations of descending order of ""bestness"", and/or to imply there is further depth to the book data in the indicated direction. If the bids & asks had been oriented side-by-side instead, they might have chosen to represent it as below, re-arranging the rows in the ""Asks"" in the opposite order (i.e. in the order you had expected) so that the ""bests"" are both in the top row:"
"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." "When people talk about ""the price"" of a stock, they usually mean one of the following: Last price:  The price at which a trade most recently took place.  If someone sold (and someone else bought) shares of XYZ for $20 each, then until another trade occurs, the last price of the stock will be quoted at $20. Bid price: The highest price at which someone is currently offering to buy the stock. Ask price: The lowest price at which someone is currently offering to sell the stock. As you can see, all of these are completely determined by the people buying and selling the stock." My broker collates the order book by price and marketplace, displaying the number of shares available at each level, sorted as in Victor's screencap. You can glean information from not just a snapshot of the order book but also by watching how it changes over time. Although it's not always a complete picture -- many brokers hold limit orders internally until the market is close, at which point they'll route to an exchange or trade internally. And of course skilled market participants know that there's people out there looking to glean information from the order book and will act to confuse the picture. The order book can show you: Combined with a list of trades (price & size, and whether it was a buy or sell), you can get a much more complete picture of what's going on with a stock than by looking at charts alone. "If there are no limit orders on the opposite side of the book when your market order gets its turn for execution, it should be rejected by the market.  A market order should generally not ""sit on the book"" like your question suggests waiting for another order to arrive.  Thus, the situation that you describe should not happen in an ordinary market that is operating in an orderly fashion. This is not to say that your order cannot ""sit"" for a while in a queue - If there is heavy volume, orders will be executed in order, so your your market order may have to wait for orders entered ahead of it to be processed.  But once its turn comes up, that's it. There are some related points to consider: I should caution that my answer is biased a bit to US stock markets, whereas you asked about currency markets.  I believe the same basic principles apply, but I'd be swayed by someone with evidence to the contrary.  I'd also note that currency tends to be more liquid than stock, so I think it's less likely that this situation would come up.  Maybe possible for a ""weak"" currency or a currency that experiences a sudden crisis of some sort." It could be that the contracts were bought at cheaper prices such as $.01 earlier in the day. What you see there with the bid and ask is the CURRENT bid and CURRENT ask. The high ask price means there is no current liquidity, as someone is quoting a very high ask price just in case someone really wants to trade that price. But as you said, no one would buy this with a better price on a closer strike price. The volume likely occurred at a different price than listed on the current ask. "I am not familiar with this broker, but I believe this is what is going on: When entering combination orders (in this case the purchase of stocks and the writing of a call), it does not make sense to set a limit price on the two ""legs"" of the order separately. In that case it may be possible that one order gets executed, but the other not, for example. Instead you can specify the total amount you are willing to pay (net debit) or receive (net credit) per item. For this particular choice of a ""buy and write"" strategy, a net credit does not make sense as JoeTaxpayer has explained. Hence if you would choose this option, the order would never get executed. For some combinations of options it does make sense however. It is perhaps also good to see where the max gain numbers come from. In the first case, the gain would be maximal if the stock rises to the strike of the call or higher. In that case you would be payed out $2,50 * 100 = $250, but you have paid $1,41*100 for the combination, hence this leaves a profit of $109 (disregarding transaction fees). In the other case you would have been paid $1,41 for the position. Hence in that case the total profit would be ($1,41+$2,50)*100 = $391. But as said, such an order would not be executed. By the way, note that in your screenshot the bid is at 0, so writing a call would not earn you anything at all." "Assuming that no one else has hit the ask, and the asks are still there, yes, you will fill $54.55 as long as you didn't exhaust that ask. Actually, there is no ""current price at which the stock is exchanging hands""; in reality, it is ""the last price traded"". The somebody who negotiated prices between buyers & sellers is the exchange through their handling of bids & asks.  The real negotiation comes between bids & asks, and if they meet or cross, a trade occurs. It's not that both bid & ask should be $54.55, it's that they were. To answer the title, the reasons why the bid and ask (even their midpoint) move away from the last price are largely unknown, but at least for the market makers, if their sell inventory is going away (people are buying heavily and they're running out of inventory) they will start to hike up their asks a lot and their bids a little because market makers try to stay market neutral, having no opinion on whether an asset will rise or fall, so with stocks, that means having a balanced inventory of longs & shorts.  They want to (sometimes have to depending on the exchange) accommodate the buying pressure, but they don't want to lose money, so they simply raise the ask and then raise the bid as people hit their asks since their average cost basis has risen. In fact (yahoo finance is great about showing this), there's rarely 1 bid and 1 ask.  Take a look at BAC's limit book: http://finance.yahoo.com/q/ecn?s=BAC+Order+Book You can see that there are many bids and many asks.  If one ask is exhausted, the next in line is now the highest.  The market maker who just sold at X will certainly step over the highest bid to bid at X*0.9 to get an 11% return on investment." While volume per trade is higher at the open and to a lesser extent at the close, the overall volume is actually lower, on average. Bid ask spreads are widest at the open and to a lesser extent at the close. Generally, bid ask spreads are inversely proportional to overall volumes. Why this is the case hasn't been sufficiently clearly answered by academia yet, but some theories are that "Honestly, I wonder if the other answerers aren't overthinking it. Their answers are detailed and correct, but what your coach may be saying is this: When you have bought a stock, on cash or margin, and you are watching it rise you are evaluating when you sell on the price of the stock you are seeing. In reality, you should look at the bid (price buyers will give you for the stock) and ask (price sellers will charge you for the stock) prices. If the stock is going up, odds are the price of the stock is very close to the ask price because it is purchases that are driving it up, but that's not what you're going to get when you sell. You're going to get something around the bid price. If the spread between the two is large (i.e. a volatile stock) this could be many cents or more lower than the ask price. Therefore, what your coach may mean by ""Selling on Ask"" is you're using the stock price when it's equal or close to the ask price to decide when to sell, instead of letting the stock peak and drop (when its price will approach the bid price) or letting the trailing bid offers catch up to your desired sell point and selling then (i.e. letting the stock point grow PAST your sell point, dragging the bid price up with it). Just a thought, but that sounds like a term a coach would come up with to mean selling and getting less than you thought you were going to from the sale. (I know it's a necro reply, but the Interwebs are immortal and people come via Google... I did)" If you want to make sure you pay at or below a specific price per share, use a limit order. If you want to buy the stock close to the current price, but aren't price sensitive, use a market order. Market orders are typically not a great idea because if you're buying thousands or tens of thousands of shares this can mean a large swing in cost if the market suddenly changes direction. "It depends on many factors, but generally, the bid/ask spread will give you an idea. There are typically two ways to buy (or sell) a security: With a limit order, you would place a buy for 100 shares at $30-. Then it's easy, in the worst case you will get your 100 shares at $30 each exactly. You may get lucky and have the price fall, then you will pay less than $30. Of course if the price immediately goes up to say $35, nobody will sell at the $30 you want, so your broker will happily sit on his hands and rake in the commission while waiting on what is now a hail Mary ask. With a market order, you have the problem you mention: The ticker says $30, but say after you buy the first 5 shares at $30 the price shoots up and the rest are $32 each - you have now paid on average $31.9 per share. This could happen because there is a limit order for 5 at $30 and 200 at $32 (you would have filled only part of that 200). You would be able to see these in the order book (sometimes shown as bid/ask spread or market depth). However, the order book is not law. Just because there's an ask for 10k shares at $35 each for your $30 X stock, doesn't mean that by the time the price comes up to $35, the offer will still be up. The guy (or algorithm) who put it up may see the price going up and decide he now wants $40 each for his 10k shares. Also, people aren't obligated to put in their order: Maybe there's a trader who intends to trade a large volume when the price hits a certain level, like a limit order, but he elected to not put in a limit order and instead watch the ticker and react in real time. Then you will see a huge order suddenly come in out of nowhere. So while the order book is informative, what you are asking is actually fundamentally impossible to know fully, unless you can read the minds of every interested trader. As others said, in ""normal"" securities (meaning traded at a major exchange, especially those in the S&P500) you simply can't move the price, the market is too deep. You would need millions of dollars to budge the price, and if you had that much money, you wouldn't be asking here on a QA site, you would have a professional financial advisor (or even a team) that specializes in distributing your large transaction over a longer time to minimize the effect on the market. With crazier stocks, such as OTC and especially worthless penny stocks with market caps of $1 mil or less, what you say is a real problem (you can end up paying multiples of the last ticker if not careful) and you do have to be careful about it. Which is why you shouldn't trade penny stocks unless you know what you're doing (and if you're asking this question here, you don't)." "I can think of the following situations in which one could see a trade occur between the visible best bid & offer: 1) on a public exchange, people have posted hidden limit orders with either bid prices above the best visible bid or offers below the best visible offer, and incoming orders have executed against this hidden liquidity[1]; 2) some orders may have been matched in dark pools which offer ""mid-point matching"" where buy and sell orders are matched using the mid-point of the best available publicly posted bid and offer as the reference price, and which executed trades are then reported to the public markets; or 3) some internalising broker has traded off exchange directly with a client and is now reporting the trade to the public as is often required. Now how exactly any of the above situations indicates that a ""trend is about to come to an end"", I do not know. [1] Exchanges often match orders on a price/visibility/time basis, whereby the orders are prioritised by price (better prices get to trade first), then by visibility (visible orders get to trade first) then by time (first come, first serve)." If you are looking to go long (buy) you would use bid prices as this is what you will be matched against for your order to be executed and a trade to go through. If you are looking to go short (sell) you would use the ask prices as this is what you will be matched against for your order to be executed and a trade go through. In your analysis you could use either this convention or the midpoint of the two prices. As FX is very liquid the bid and ask prices would be quite close to each other, so the easiest way to do your analysis is to use the convention I listed above. "The ""price"" is the price of the last transaction that actually took place. According to Motley Fool wiki: A stock price is determined by what was last paid for it. During market hours (usually weekdays from 9:30AM-4:00PM eastern), a heavily traded issue will see its price change several times per second. A stock's price is, for many purposes, considered unchanged outside of market hours. Roughly speaking, a transaction is executed when an offer to buy matches an offer to sell. These offers are listed in the Order Book for a stock (Example: GOOG at Yahoo Finance).  This is actively updated during trading hours. This lists all the currently active buy (""bid"") and sell (""ask"") orders for a stock, and looks like this:  You'll notice that the stock price (again, the last sale price) will (usually*) be between the highest bid and the lowest ask price. * Exception: When all the buy or sell prices have moved down or up, but no trades have executed yet." It seems that any news article headline that includes a dollar amount or a percentage is click bait. In the best cases they're sensationalizing a mundane study. In the worst cases the study is as flawed as the headline. In either case the liberal arts major has their revenge on the world.