Basis refers to the price difference between the spot and futures, the spot price minus the futures price is equal to the difference between the groups (ie Basis = spot price - futures price). Under normal market conditions the base difference is negative. Usually due to the time and storage costs to produce. Buy investment costs incurred by the subject matter coupled with the holding period of the commodities or commodity-related costs all in one place. As a reference to calculate the capital gain or loss. The same point, a time difference between the spot price and the futures market in the futures price reflects the difference between the different periods of the commodity-form, quality, location and other factors.