Period arbitrage is an operation mode of establishing equal trading positions in different contract months of the same futures product and ending the transaction by hedging or delivery. The purpose is to earn the spread by observing the fluctuation of the spread between futures contracts. In the forward market, the spread is negative, showing the premium in the distant month; in the reverse market, the spread is positive, showing the premium in the recent month. Generally speaking, the price difference (absolute value) consists of the holding cost (or position fee), that is, the storage fee, insurance premium and interest paid for owning or holding warehouse receipts or positions.