I obviously do not trade futures and have no plans on trading futures but was looking at a buy order for the ES in my TOS account and noticed that I would not have to initially have to put up any capital when buying a contract. My question is: Do you have to put up a specific amount of money to buy a contract or do you just have to settle for the difference in the price when you sell the contract? An example: You buy contact X at $100. The price goes down and you sell at $90. Do you have to initially put up margin to hold the contract or just settle for the $10 when you sell it? Obviously this is a really easy question but I couldn't really find an answer for it yet.