Used in association with the EQUITABLE SHARING MECHANISM. A period during which the current take-or-pay liabilities of the pipeline were incurred. A customer's cumulative deficiency in purchases during this period, in comparison to a BASE PERIOD, is compared to the system's total cumulative deficiencies to determine that customer's proportionate share of fixed take-or-pay charges. See BASE PERIOD, EQUITABLE SHARING MECHANISM and PURCHASE DEFICIENCY METHODOLOGY.