Mont. Code Ann. § 7-7-2211

The term "serial bonds", as used in this part, means a bond issue payable in annual installments of principal commencing not more than 2 years from the date of issue, any one installment consisting of one or more bonds, with the principal amount of bonds maturing or subject to mandatory sinking fund redemption in each installment, commencing with the installment payable in the fourth year after the date of issue, not exceeding three times the principal amount of the bonds payable in the immediately preceding installment.