Bid-Ask at market open, which comes first? [duplicate]
"When you place a limit sell order of $10.00 (for a stock on an option) you are adding your order to the book.  Anyone who places a buy at-the-market or with a limit price over $10.00 will have that order immediately fulfilled through the offer you have placed on the book. On the other hand, if that other person places a buy for $8.00, then the spread will now be ""$8.00 bid, $10.00 ask"". Priority is based on first the price (all $9.99 asks will clear before $10.00) and within each bucket this is based on the time your order was submitted. This is why in bidding markets (including eBay) buying at $x.01 is way better than $x.00 and selling at $x.99 is better than $(x+1).00. Source: https://en.wikipedia.org/wiki/Order_(exchange) under ""first-come-first-served""" The options market requires much more attention to avoid the situation you're describing. An overnight $10 ask will remain on the books most likely as Good-Til-Canceled.  The first to bid the low order gets it.  If traders are paying attention, which they probably are then they will bid at $10.  If not, they will bid immediately at $20. If they crossed the order, it would be filled at their higher than $10 bid. This is all governed by the exchange where the ask is posted, and most implement price-time priority.
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. "First, as @littleadv mentions, and as I've pointed out before, anyone who participates in a market using limit orders (which, by the way, should be every non-professional investor) is by definition a market maker.  So, I will assume that your question pertains both to official market makers and to ""retail investors"" using limit orders. When you remark that there are such ""tight spreads"" in ""liquid assets"", what you are really saying is ""wow, look at all the market makers in these products!""  That's the benefit of electronic trading and algorithmic traders -- millions of participants each with their own opinion of the value of a financial instrument, trying to find people who have very specifically opposing opinions of the value of that same instrument.  This is called price discovery, and is the entire point of financial markets. So, you ask why are there all these market makers present to create such tight spreads in assets like SPY?  Answer: Because they can make money in these markets: Imagine (towards a contradiction) that market makers thought they couldn't make money by offering tight spreads in SPY, and so SPY had a wider spread than it actually does.  For example, say the highest bid for SPY was $99.98 and the lowest ask was $100.01.  Now imagine that a market maker with perfect knowledge of the future came along knowing that he would be able to sell SPY for $100.01 in 5 minutes.  Then he would load up as many buy orders as he could for $100.00 or lower.  (He wouldn't bid $100.01 or higher because those trades would not be profitable according to his information -- at least not 5 minutes from now.)  So the spread had previously been $0.03 and then suddenly it was $0.01, all because a market maker with better information came along and realized he could make money by creating a tighter market! Now, nobody has perfect knowledge of the future, which is why markets are never infinitely tight or infinitely liquid.  Each market maker has to weigh possible profits against the probability that those profits will actually turn into losses.  But if one market maker decides not to participate in a particular instrument, there's bound to be another market maker who will happily take his place.  So the very fact that there are so many market participants with resting buy/sell orders for SPY right now is proof that there are market makers able to make money doing so.  If they could not make money, they wouldn't be there, and the spread would be wider. 10-15 years ago, before electronic trading and algorithmic trading, the number of market participants was far lower, and the spreads were far wider, meaning retail investors like you and me had a much harder time making money.  The only people making money were the institutional investors, the brokers, and the exchanges.  Now that all these new millions of players are present in the market, retail investors like you and me get to participate and make money too." "You can choose to place successively lower buy limit orders, but whether they get filled or not is not a given; it depends on whether sellers care to accept your bid. In your example of a 49.98 / 50.01 spread, if you place a buy with limit of 49.99, it won't get filled (if the order reaches the market while still at 49.98 / 50.01) immediately, but will be added to the order book.  By being added to the order book, the markets bid and ask become 49.99 / 50.01.  Your order won't get filled until some seller places a market order or a sell limit order of 49.99 or less.  No guarantee that that will happen, and even if it does, there's nothing to say that your follow-up buy at 49.98 will ever be filled.  In fact, your 49.98 buy order queues up at the ""end of the line"" behind all previously pending 49.98 bids, since your order arrived after those other bids.  Since the initial conditions you supposed had a 49.98 bid, such an order exists (or at least did exist; it might have been cancelled in the intervening moment. Basically, your first buy at 49.99, if it happens, has essentially no influence on whether your second buy at 49.98 will happen.  You can't expect to move the market lower by making a bid that is higher (49.99) than the existing best bid (49.98).  Whatever influence your 49.99 order has is to raise the market's price, not lower it." 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. "The Auction Market is where investors such you and me, as well as Market Makers, buy and sell securities.  The Auction Markets operate with the familiar bid-ask pricing that you see on financial pages such as Google and Yahoo.  The Market Makers are institutions that are there to provide liquidity so that investors can easily buy and sell shares at a ""fair"" price. Market Makers need to have on hand a suitable supply of shares to meet investor demands.  When Market Makers feel the need to either increase or decrease their supply of a particular security quickly, they turn to the Dealer Market. In order to participate in a Dealer Market, you must be designated a Market Maker.  As noted already, Market Makers are dedicated to providing liquidity for the Auction Market in certain securities and therefore require that they have on hand a suitable supply of those securities which they support.  For example, if a Market Maker for Apple shares is low on their supply of Apple shares, then will go the Dealer Market to purchase more Apple shares.  Conversely, if they are holding what they feel are too many Apple shares, they will go to the Dealer Market to sell Apple shares.  The Dealer Market does operate on a bid-ask basis, contrary to your stated understanding. The bid-ask prices quoted on the Dealer Market are more or less identical to those on the Auction Market, except the quote sizes will be generally much larger. This is the case because otherwise, why would a Market Maker offer to sell shares to another Market Maker at a price well below what they could themselves sell them for in the Auction Market.  (And similarly with buy orders.)  If Market Makers are generally holding low quantities of a particular security, this will drive up the price in both the Dealer Market and the Auction Market. Similarly, if Market Makers are generally holding too much of a particular security, this may drive down prices on both the Auction Market and the Dealer Market." "The two dimensions are to open the trade (creating a position) and to buy or sell (becoming long or short the option). If you already own an option, you bought it to open and then you would sell it to close. If you don't own an option, you can either buy it to open, or sell it (short it) to open. If you are already short an option, you can buy it back to close. If you sell to open covered, the point is you're creating a ""covered call"" which means you own the stock, and then sell a call. Since you own the stock, the covered call has a lot of the risk of loss removed, though it also subtracts much of the reward possible from your stock." The simple answer: The opening price is the price of the first trade of the day and the closing price is the price of the last trade of the day.  And since the stock price change from trade to trade they are usually different. "Stock price is set to the price with the highest transaction volume at any given time.  The stock price you cited was only valid in the last transaction on a specific stock exchange. As such it is more of an ""historic"" value. Next trade will be done with the next biggest volume. Depending on the incoming bids and asks this could be higher or lower, but you can assume it will not be too far off if there is no crash underway. Simple example stock exchange:"