Futures varieties include far-month contracts and near-month contracts. Generally, one or two relatively active contracts among several contracts are the main contracts of this variety. The main contracts have a large trading volume and relatively large fluctuations, and are generally relatively recent contracts, such as the gold 0806 contract, which is the current main contract. The contract price is different in different months. If the contract price gap in recent months is too large, then the market will have the opportunity to arbitrage across periods. For example, gold 0806 contract 220 yuan/gram, gold 0807 contract 230 yuan/gram, then you can buy in 0806 contract and sell the same number of warehouse receipts in 0807 contract to achieve arbitrage.