<b>Priority (time), Parity (ability to fulfill), and Precedence (size).</b>
Order placement would be as follows:
If there were three traders with the following orders on the book; Trader A has 300 shares to sell and he has time priority, Trader B has 500 shares for sale and trader C has 1000 shares for sale. A contra order to buy comes for 100 shares. A gets the fill since he had time priority and the ability to fill the order. Now there is no time priority since an order has been fulfilled. Another contra order comes for 300 shares. Trader A is out of the queue since he does not have parity with only 200 shares to sell. Trader B has the ability to fulfill the order but so does Trader C. Back when I was trading, all brokers carried what they called a 'lucky' coin. The two brokers would flip the coin. This was called "matched and lost" and reported to the customer if your broker lost the match. Lets say Trader B's broker won the match and he sold 300 shares. Now we have Trader A with 200 shares, Trader B with 200 shares and Trader C still with 1000 shares to sell. Now another contra order comes in for 200 shares. Trader C will get a fill on this order since all three have the ability to fulfill and none of them have time priority but Trader C has precedence with size. This was the method used on the floor. Now with electronic trading, of course brokers cannot toss a coin. I have watched orders being filled out of the queue and only time and precedence appears to be used and size determines fulfillment.
It is not unusal for markets not to inform the public of order fulfillment procedure. If anyone can obtain the current method of order fulfillment on the electronic markets, please share it with us. To be fair, in my opinion, orders should be filled strictly on time priority. But I have tested the market and watched the screens enough to know it's not.
One good thing about the electronic market, limit orders cannot by passed on a fast market without an execution. On the floor, a specialist could bypass orders on the book without cleaning up the book with the approval of a floor governor.
Order placement would be as follows:
If there were three traders with the following orders on the book; Trader A has 300 shares to sell and he has time priority, Trader B has 500 shares for sale and trader C has 1000 shares for sale. A contra order to buy comes for 100 shares. A gets the fill since he had time priority and the ability to fill the order. Now there is no time priority since an order has been fulfilled. Another contra order comes for 300 shares. Trader A is out of the queue since he does not have parity with only 200 shares to sell. Trader B has the ability to fulfill the order but so does Trader C. Back when I was trading, all brokers carried what they called a 'lucky' coin. The two brokers would flip the coin. This was called "matched and lost" and reported to the customer if your broker lost the match. Lets say Trader B's broker won the match and he sold 300 shares. Now we have Trader A with 200 shares, Trader B with 200 shares and Trader C still with 1000 shares to sell. Now another contra order comes in for 200 shares. Trader C will get a fill on this order since all three have the ability to fulfill and none of them have time priority but Trader C has precedence with size. This was the method used on the floor. Now with electronic trading, of course brokers cannot toss a coin. I have watched orders being filled out of the queue and only time and precedence appears to be used and size determines fulfillment.
It is not unusal for markets not to inform the public of order fulfillment procedure. If anyone can obtain the current method of order fulfillment on the electronic markets, please share it with us. To be fair, in my opinion, orders should be filled strictly on time priority. But I have tested the market and watched the screens enough to know it's not.
One good thing about the electronic market, limit orders cannot by passed on a fast market without an execution. On the floor, a specialist could bypass orders on the book without cleaning up the book with the approval of a floor governor.