A trade blotter is a record of each trade that transacted for a given period of time, normally one day.
A blotter would include the time of the trade, the ECN or dark pool market the trade occurred over, the quantity, the exact price, and if it was marked as a buy, sell, or short order. Sometimes a blotter will include orders that were entered but were cancelled before they transacted as well.
A trader will use a blotter as a way to review his trading day. The blotter allows the trader to see exactly what happened over the course of a day. It is a good idea for day traders to review their blotters at the end of every trading day, and record their observations in a trading journal.
A trader will look for places where he could have had better timing with entries or exits, or could have entered orders more efficiently, for instance by using an ECN that has a lower cost or rebate.
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A trade blotter is a valuable tool for a trader when he reviews it daily to improve his trading technique.