As a near contract (say,September 98) matures,a distant contract (i.e.,September 99) will be added on a rolling basis.

• 随着时间的经过，“近”的合约先到期，“远”的合约迟些到期。到期时，所有尚未“盖盘”的合约都要结算、交割清楚，之后该合约也就不复存在了。