Mont. Code § 7-7-2210
The term "amortization bonds", as used in this part, means that form of bond on which a part of the principal is required to be paid each time that interest becomes due and payable. The part payment of principal increases with each following installment in the same amount that the interest payment decreases, so that the combined amount payable on principal and interest is the same on each interest payment date. However, the payment on the initial interest payment date may be less or greater than the amount of other payments on the bond, reflecting the payment of interest only or the payment of interest for a period different from that between other interest payment dates. The final payment may vary in amount from the other payments to the extent resulting from rounding amounts in the other payments.
§ 7-7-2210, MCA