(a) Forty-five day tolerance provision. An index that is a narrow-based security index that becomes a broad-based security index for no more than 45 business days over 3 consecutive calendar months shall be a narrow-based security index.

(b) Transition period for indexes that cease being narrow-based security indexes for more than forty-five days. An index that is a narrow-based security index that becomes a broad-based security index for more than 45 business days over 3 consecutive calendar months shall continue to be a narrow-based security index for the following 3 calendar months.

(c) Trading in months with open interest following transition period. After the transition period provided for in paragraph (b) of this section ends, a national securities exchange may continue to trade only in those months in the security futures product that had open interest on the date the transition period ended.

(d) Definition of calendar month. Calendar month means, with respect to a particular day, the period of time beginning on a calendar date and ending during another month on a day prior to such date.