Basis is the difference between the spot price and the futures price of a specific commodity at a specific time and place. Its calculation method is the spot price minus the futures price. If the spot price is lower than the futures price, the basis is negative; The spot price is higher than the futures price and the basis is positive. The connotation of basis is determined by the difference between transportation cost and holding cost between spot market and futures market. In other words, the basis includes two components: time and space, and the transportation cost reflects the time factor between the spot market and the futures market. That is, the holding cost between two different delivery months reflects the holding cost or preservation cost of a commodity from one time period to another, including storage space, interest, insurance premium and so on. Storage cost is the actual expenditure paid for storing goods, which generally changes with time and region; Interest is the capital cost needed to store goods, and the interest cost will change with the increase of interest rate; Insurance cost is the cost of storing goods for insurance. The basis reflecting the holding cost changes with time; The longer the time, the greater the holding cost. Since the futures contract is only deliverable, the seller should deliver the goods to the buyer after the expiration.

Theoretically, the futures price is the market's forecast of the future spot market price, and there is a close relationship between them. Due to the similarity of influencing factors, futures prices and spot prices often show a relationship of ups and downs; However, the influencing factors are not exactly the same, so the changes of the two are not completely consistent, and the relationship between spot price and futures price can be described by basis. Basis is the difference between the spot price of a commodity in a specific place and the price of a specific futures contract of the same commodity, that is, basis = spot price-futures price. The basis is sometimes positive (called the inverse market) and sometimes negative (called the positive market). Therefore, the basis is a dynamic indicator of the actual operation change between the futures price and the spot price.

The change of basis directly affects the effect of hedging. From the principle of hedging, it is not difficult to see that hedging actually replaces the risk of price fluctuation in the spot market with basis risk. Therefore, in theory, if the basis remains unchanged at the beginning and end of hedging, it is possible to achieve complete hedging. Therefore, the hedger should pay close attention to the change of basis and choose favorable opportunity to complete the transaction.

At the same time, the fluctuation of basis is relatively stable than that of futures price and spot price, which provides favorable conditions for hedging transactions; Moreover, the change of basis is mainly controlled by holding cost, which is much more convenient than directly observing the change of futures price or spot price.

When the hedger fails to find a futures contract that completely matches the spot position in terms of variety, duration and quantity, when choosing a substitute contract for hedging operation, the basis risk arises because the cash flow cannot be completely locked.