The master contract is generally formed by beans 0 1, 05, 09. The main contract is a contract in which both long and short parties are interested. It has something to do with the seasonal cycle of the product.

The main contract is changeable. 05, with the passage of time, 1009 will become the main contract. Generally there is only one main contract at the same time. In a short time, there will be two major contracts.

However, industrial products are different. Every month, they will become the main contracts, such as metals. Generally, the contract is based on the month of delivery near 4 months. Take a step back every month. In other words, everyone is interested in how long and how short the contract will be delivered after three months. Speculative funds like to enter the market only when the transaction volume is large.

In foreign countries, what we usually call London copper is actually futures copper delivered in three months.