How does a limit order work for a credit spread?
As you probably know, a credit spread involves buying a call (or put) at one strike and selling another call (or put) at another with the same maturity, so you're dealing with two orders. Your broker will likely have to fill this order themselves, meaning that they'll have to look at the existing bid/asks for the different strikes and wait until the difference matches (or exceeds) your limit order.  Obviously they can't place limit orders on the legs individually since they can't guarantee that they will both be executed.  They also don't care what the individual prices are; they just care what the difference is.  It's possible that they have computer systems that examine existing bids and asks that would fill your order, but it's still done by the broker, not the exchange.  The exchange never sees your actual limit order; they will just see the market orders placed by your broker.
"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." Believe it or not, this is done as a service to you. The reason for this has to do with a fundamental difference between a credit card account and a checking account.  With a credit card account, there is no money in the account; every charge is borrowed money. When you get to your credit limit, your credit transactions will start getting declined, but if the bank does for some reason let one get approved, it's not a big deal for anyone; it just means that you owe a little more than your credit limit. Note that (almost) every credit card transaction today is an electronic transaction.  A checking account, however, has real money in it. When it is gone, it is gone. When a balance inquiry is done, the bank has no way of knowing how many checks you've written that have not been cashed yet. It is a customer's responsibility to know exactly how much money is available to spend.  If you write more checks than you have money for in your account, technically you have committed a crime. Unfortunately, there are too many people now that are not taking the responsibility of calculating their own checking account balance seriously, and bad checks are written all the time. When a bank allows these transactions to be paid even though you don't have enough money in your account, they are preventing a crime from being committed by you. The fee is a finance charge for loaning you the money, but it is also there to encourage you not to spend more than you have.  Even if you use a debit card, it is still tied to a checking account, and the bank doesn't know if you have written enough checks to overdraw your account or not. It is still your responsibility to keep track of your own available balance.  Every time this happens to you, thank the bank as you pay this fee, and then commit to keeping your own running balance and always knowing how much you have left in your account. As others have stated, the current price is simply the last price at which the security traded. For any given tick, however, there are many bid-ask prices because securities can trade on multiple exchanges and between many agents on a single exchange. This is true for both types of exchanges that Chris mentioned in his answer. Chris' answer is pretty thorough in explaining how the two types of exchanges work, so I'll just add some minor details. In exchanges like NASDAQ, there are multiple market makers for most relatively liquid securities, which theoretically introduces competition between them and therefore lowers the bid-ask spreads that traders face. Although this results in the market makers earning less compensation for their risk, they hope to make up the difference by making the market for highly liquid securities. This could also result in your order filling, in pieces, at several different prices if your brokerage firm fills it through multiple market makers. Of course, if you place your order on an exchange where an electronic system fills it (the other type of exchange that Chris mentioned), this could happen anyway.  In short, if you place a market order for 1000 shares, it could be filled at several different prices, depending on volume, multiple bid-ask prices, etc. If you place a sizable order, your broker may fill it in pieces regardless to prevent you from moving the market. This is rarely a problem for small-time investors trading securities with high volumes, but for investors with higher capital like institutional investors, mutual funds, etc. who place large orders relative to the average volume, this could conceivably be a burden, both in the price difference across time as the order is placed and the increased bookkeeping it demands.  This is tangentially related, so I'll add it anyway. In cases like the one described above, all-or-none (AON) orders are one solution; these are orders that instruct the broker to only execute the order if it can be filled in a single transaction. Most brokers offer these, but there are some caveats that apply to them specifically. (I haven't been able to find some of this information, so some of this is from memory).  All-or-none orders are only an option if the order is for more than a certain numbers of shares. I think the minimum size is 300 or 400 shares.  Your order won't be placed until your broker places all other orders ahead of it that don't have special conditions attached to them.  I believe all-or-none orders are day orders, which means that if there wasn't enough supply to fill the order during the day, the order is cancelled at market close.  AON orders only apply to limit orders. If you want to replicate the behavior of a market order with AON characteristics, you can try setting a limit buy/sell order a few cents above/below the current market price. "There are a number  of federal tax deductions and credits available for education expenses.  They are too numerous to describe here, but the place to get full details is IRS Pub 970.  Note that many, but not all, of them require that you be enrolled in a degree program; since this does not seem to be the case for you, you would not be eligible for those programs. None of them is as simple / generous as ""deduct the full amount of your tuition with no limits"".  Also note that there are restrictions on using more than one of these deductions or credits in any given tax year. You might pay special attention to Chapter 12, ""Business Deduction for Work-Related Education"".  In particular, this program allows you to deduct transportation expenses under some conditions, which does not seem to be the case for the other programs.  But also note carefully the restrictions.  In particular, ""Education that is part of a program of study that will qualify you for a new trade or business is not qualifying work-related education.""  So if you are not already working in the field of IT, you may not be eligible for this deduction." I too was very confused when I tried to be tricky and paid down my balance BEFORE the bill date. I thought this would be a great thing because it would show my utilization near zero percent. The opposite happen, it dropped my credit score from 762 to 708. Here is the best example I can come up with when it comes to utilization. Lets pretend you are an insurance company and you trying to figure out who are the best risk drivers. The people that drive 10% of the day are a better risk than the people that drive 50% of the day. The people that drive 50% of the day are a better risk than the people that drive 90% of the day. Here is the rub when people drive 0%. When you look at the people at 0% they appear to be walking, busing or flying. What they are NOT doing is driving. Since they are not driving (using Credit) they are viewed as POOR drivers since they are not keeping up on their driving skills. (Paying bills, watching how they spend, and managing their debt). So, now before the billing date I pay down my balance to something between 5 to 10% of my utilization. After the bill is issued, I pay it off in FULL. ( I am not going to PAY these crazy interest rates). What shows up on my credit report is a person that is driving his credit between 5 and 10% utilization. It shows I know I how to manage my revolving accounts. I know it's dumb, you would think they reward people that have zero debt, I don't hate banks I hate the game. ( I do love me some reward points =)) The Limit Order are matched based on amount and time. The orders are listed Highest to Lowest on the Buy Side. The orders are listed Lowest to Highest on the Sell Side. If there are 2 Sell orders for same amount the order which is first in time [fractions of milliseconds] is first.  The about is the example as to how the orders would look like on any exchange.  Now the highest price the buyer is ready to pay is 20.21 and the lowest price a seller is ready to sell for is 20.25. Hence there is no trade. Now if a new Buy order comes in at 20.25, it matches with the sell and the deal is made. If a new Buy order comes in at 20.30, it still matches at 20.25. Similarly if a Sell order come in at 20.21, it matches and a deal is made. If a Sell order come in at 20.11, it still matches 20.21.   Incase of market order, with the above example if there is a Buy order, it would match with the lowest sell order at 20.25, if there is not enough quantity , it would match the remaining quantity to the next highest at 20.31 and continue down. Similarly if there is a Sell market order, the it would match to the maximum a seller is ready to buy, ie 20.21, if there is not sufficient buy quantity at 20.21, it will match with next for 20.19   If say there are new buy order at 20.22 and sell orders at 20.24, these will sit first the the above queue to be matched. In your above example the Lowest Sell order was at 20.10 at time t1 and hence any buy order after time t1 for amount 20.10 or greater would match to this and the price would be 20.10. However if the Buy order was first ie at t1 there was a buy order for 20.21 and then at time later than t1, there is a sell order for say 20.10 [amount less than or equal to 20.21] it would match for 20.21. Essentially the market looks at who was the first to sell at lower price or who was the first to buy at higher price and then decide the trade.  Edit [To Clarify xyz]: Say if there is an Sell order at $10 Qty 100. There is a buyer who is willing to pay Max $20 and is looking for Qty 500. Your key assumption that the Buyer does not know the current SELL price of $10 is incorrect. Now there are multiple things, the Buyer knows the lowest Sell order is at $10, he can put a matching Buy order at $10 Qty 100, and say $11 Qty 100 etc. This is painful. Second, lets say he puts a Buy order at $10 Qty 100, by the time the order hits the system someone else has put the trade at $10 and his order is fulfilled. So this buyer has to keep looking at booking and keep making adjustments, if its a large order, it would be extremely difficult and frustrating for this Buyer. Hence the logic of giving preference. The later Buy order says ... The Max I can pay is $20, match eveything at the current price and get the required shares. Then we should create a derivative that allows people to bet on the bond failing, and package tons of them into securities, which you can swap for cash. You're swapping a bet on someone defaulting on their credit... Swapping a credit default... Hmm... I don't see this going wrong in any way. But how does the quantity matching happen? For example, if I want to buy 1000 shares at $100, but there is only one seller to sell 10 shares at $100, what happens then? This depends on the type of order you've placed. If you placed a fill-or-kill order, your order to buy or sell a certain number of shares is routed to the trading floor for immediate execution. If the order cannot be immediately filled, it is cancelled (killed) automatically. Note that the order must be filled in its entirety. Partial fills are not allowed. In your example, your buy order wouldn't be filled because it couldn't be matched to a sell order of the same volume. This is similar to an all-or-none order, which is an order that contains  A condition instructing the broker to fill the order completely or not at all. If there is insufficient supply to meet the quantity requested by the order then it is canceled at the close of the market. In this case, if your order wasn't matched to an order of the same volume by the time the market closes, it's cancelled. If you simply placed a market/limit order, and (in the case of the limit order), part of your order was matched to another order with the right price, that part of your order will be filled, while the rest will remained unfilled.