Mont. Code Ann. § 72-11-103

The degree of lineal kinship of two persons is computed by counting one degree for each person in the line of ascent or descent, exclusive of the person from whom the computing begins. The child is, with regard to the parent, in the first degree; the grandchild is, with regard to the grandparent, in the second degree; and vice versa as to the parent and grandparent with regard to their respective children and grandchildren.