Do marketmakers always quote a bid and ask simultaneously
"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, but also note each exchange have rules that states various conditions when the market maker can enlarge the bid-ask (e.g. for situations such as freely falling markets, etc.) and when the market makers need to give a normal bid-ask. In normal markets, the bid-asks are usually within exchange dictated bounds.  MM's price spread can be larger than bid-ask spread only when there are multiple market makers and different market makers are providing different bid-asks. As long as the MM under question gives bid and ask within exchange's rules, it can be fine. These are usually rare situations. One advice: please carefully check the time-stamps. I have seen many occasions when tick data time-stamps between different vendors are mismatched in databases whereas in real life it isn't.  MM's profits not just from spreads, but also from short term mean-reversion (fading). If a large order comes in suddenly, the MM increases the prices in one direction, takes the opposite side, and once the order is done, the prices comes down and the MM off-loads his imbalance at lower prices, etc.
"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." "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)" "According to Yahoo, AAPL was trading at $113.26 at 1:10 PM on 11/13/15, which is the approximate time of your option quote.  You provided a quote for AAPL at 4:15, and the stock happened to keep going down most of the that afternoon.  To make a sensible comparison, you need to take contemporary prices on both the stock and the option.  The quote on the option also shows the ""price"" being outside of the bid-ask range, which suggests that the option was trading thinly and that the last price occurred sometime earlier in the day.  If you use a price in the bid-ask range ($21.90-$22.30) and use the price of AAPL at the time of the put quote, you'll come up with a price that's much closer to your expectation." 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. No, it's not even remotely accurate in the current sense. Both markets have counterparties directly executing against one another, and both have auctions. The auction mechanics are different (with NYSE's Specialist/DMM model) but during normal market hours there isn't much difference. "I can address what it means to ""pick off"" all those trades... As quantycuenta & littleadv have said, it is absolutely true that professionals ""prey"" on less-sophisticated market participants. They aren't in the market for charity's sake.  If you're not familiar with the definition of the word ""arbitrage"", look it up.  One possible strategy that can be employed with HFT machinery in order to arbitrage successfully in the stock market is to 'intercept' orders that are placed on various exchanges. In order to do this, an HFT organization watches all the transactions at once to find opportunities to buy low and sell high.  A good explanation of it is described here in this NY Times article; I'll paraphrase what that article lays out.  Stocks are traded through multiple exchanges The first key point to understand is that stocks listed on one exchange (i.e. the NYSE) can be sold on multiple exchanges. That's where the actual ""I would like to sell 100 shares of Ford stock"" is matched with ""I would like to buy 100 shares of Ford stock."" There are multiple clearinghouses on the various exchanges. Your order gets presented to one exchange at a Time An ideal market maker would like to look at the order books for a given stock, say Ford, and see that in exchange A there's a sell order for 100 shares of F at $15.85, and in exchange B there's a buy order for 100 shares of F at $15.90.  Arbitrage Market maker buys from A, sells in B, and pockets $0.05 * 100... $5. It's not much, but it was relatively risk free. Also, scale this up to the scale of the US' multiple stock exchanges, and there are lots of opportunities to make $5 every second.  Computers are (of course) faster than people To tie it in completely with your question about 'picking off trades', HFT rigs can be set up and programmed to go faster than an average retail investor's order.  Let's say you execute the trade to buy 100 shares @ $15.85 as a retail investor. The HFT rigs see your order starting to make the rounds of the different exchanges that your brokerage works through, and go out in front in a matter of milliseconds, finding the orders that are less than $15.85 and less than or equal to 100 shares. They execute a transaction, buy them up, sell to you, and pocket the difference.  You have been ""picked off"".  It's admittedly not the only way to use HFT equipment to make money, but it's definitely one way to do it." I don't have all the answers. On a illiquid stock, such situations do arise and there are specific mechanisms used by exchanges to match the order. It is generally not advisable to use market order on illiquid stock. There are lots of different variations here. I guess this comes down to specifications for individual exchanges, but I'm wondering if there's a standard here or a way to approach it from basic rules that clears up all these situations. There are quite a few variations and different treatments. Market order that are placed when the market is closed or just around market opening are traded at Market Open price that each exchange has a formulae to calculate. In the process Market Buy are matched to Market Sell at the Exchange calculated price. Not all order get matched and there could be spill over's. These are then matched to limit orders. Is this determined based on which sell order came first, or based on which would result in the best deal for the incoming buyer? Generally Market orders have highest priority of execution. Each goes to a different agency. Yes, it is normal that the lender queries more than one agency.