The main contract is usually valid for 3~4 months. Near the final delivery, the main institution will start to transfer its position to the second main contract, and then the second main contract will become the new main contract.

The difference between the main contract and the sub-main contract: The main difference lies in the different trend strength, that is, in the same time period, the main contract and the sub-main contract have different rising or falling ranges. Under normal circumstances, the main contract has a large decline in the down market, and the secondary main contract has a large increase in the up market. In addition, the positions and turnover of the main contract are significantly higher than other contracts. However, with the approach of the delivery of the main contract, the difference between positions and turnover gradually narrowed, and the positions and turnover of the second main contract gradually overwhelmed the main contract and became the new main contract.