Suppose I buy a futures contract and later, before expiry date, I sell it out. Is something physically exchanged between myself and the buyer, e.g. a paper certificate specifying the contract's details? If not, what is it that changes to reflect this transaction? Is it the table row that identifies this contract in the futures exchange's database that is updated with the new buyer's name replacing my name in the "buyer" column? Or perhaps it is not actually possible to sell a futures contract or change its details once it is established, and the closest you can get is to emulate the effect of selling it by entering into a completely new futures contract, but this time in the role of the seller?