The main contract position is changed to 1, 5, 9 months. The main contract refers to the contract with the largest position. The life cycle of futures is limited, futures have a delivery date, and the distance between different contracts and delivery date is different, so the trading volume of different contracts is also different. For general agricultural futures, the main contract only changes between 1, 5 and 9 months. In the case of metal futures, the main contract is to change the month from month to month, but generally February will be skipped, and sometimes August will be skipped.