Changing the price in a limit order
This depends on the stock exchange in question. Generally if you modify an existing order [including GTC], these are internally treated as Cancel/Replace Orders. Depending on the action, you may lose the time priority position and a new position would get assigned. More here. (f) Cancel/Replace Orders. Depending on how a quote or order is   modified, the quote or order may change priority position as follows: (1) If the price is changed, the changed side loses position and is   placed in a priority position behind all orders of the same type   (i.e., customer or non-customer) at the same price. (2) If one side's quantity is changed, the unchanged side retains its   priority position. (3) If the quantity of one side is decreased, that side retains its   priority position. (4) If the quantity of one side is increased, that side loses its   priority position and is placed behind all orders of the same type at   the same price.
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. You would place a stop buy market order at 43.90 with a stop loss market order at 40.99 and a stop limit profit order at 49.99. This should all be entered when you place your initial buy stop order. The buy stop order will triger and be traded once the price reaches 43.90or above. At this point both the stop loss market order and the stop limit profit order will become active. If either of them is triggered and traded the other order will be cancelled automatically. In the stock market many participants enter orders that are not necessarily set at the current market price of the stock (i.e. they are not market orders, they are limit orders). They can be lower than the market price (if they want to buy) or they can be higher than the market price (if they want to sell). The set of orders at each point of time for a security is called the order book. The lowest selling price of the order book is the offer or ask, the higher buying price is the bid. The more liquid is a security, the more orders will be in the order book, and the narrower will be the bid-ask spread. The depth of the order book is the number of units that the order book can absorb in any direction (buy or sell). As an example: imagine I want to buy 100 units at the lowest offer, but the size of the lowest offer is only 50 units, and there is not any further order, that means the stock has little depth. Limit books are managed by exchanges. If an order is not immediately filled, it is sent to the book.  From there, orders are generally executed on price-time-priority.  The one major exception is the precedence hide-not-slide orders have over earlier placed visible slidden limit orders since unslidden orders are treated like a modification/cancellation. To an exchange, a modification is the same as a cancellation since it charges no fees for placing or canceling orders, only for trades.  The timestamp is reset, and price-time-priority is applied in the same way, so if a modified order isn't immediately filled, it is sent back to the book to be filled in order of price-time-priority. "Market orders do not get priority over limit orders.  Time is the only factor that matters in price/time order matching when the order price is the same. For example, suppose the current best available offer for AAPL is $100.01 and the best available bid is $100.00.  Now a limit buy for $100.01 and a market buy arrive at around the same instant.  The matching engine can only receive one order at a time, no matter how close together they arrive. Let's say that by chance the limit buy arrives first.  The engine will check if there's a matching sell at $100.01 and indeed there is and a trade occurs.  This all happens in an instant before the matching engine ever sees the market buy.  Then it moves on to the market buy and processes it accordingly. On the other hand, let's say that by chance the market buy arrives first.  The engine will match it with the best available sell (at $100.01) and a trade occurs.  This all happens in an instant before the matching engine ever sees the limit buy.  Then it moves on to the limit buy and processes it accordingly. So there's never a comparison between the two orders or their ""priorities"" because they never exist in the system at the same time.  The first one to arrive is processed first; the second one to arrive is processed second." 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. Assuming cell A1 contains the number of trades: will price up to A1=100 at 17 each, and the rest at 14 each. The key is the MAX and MIN. They keep an item from being counted twice. If X would end up negative, MAX(0,x) clamps it to 0. By extension, if X-100 would be negative, MAX(0, X-100) would be 0 -- ie: that number doesn't increase til X>100. When A1=99, MIN(a1,100) == 99, and MAX(0,a1-100) == 0. When A1=100, MIN(a1,100) == 100, and MAX(0,a1-100) == 0. When A1=101, MIN(a1,100) == 100, and MAX(0,a1-100) == 1. Of course, if the 100th item should be $14, then change the 100s to 99s. "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."