The module is finitely presentable; a presentation matrix for this module is called the Alexander matrix. If the number of generators, "r", is less than or equal to the number of relations, "s", then we consider the ideal generated by all "r" by "r" minors of the matrix; this is the zero'th Fitting ideal or Alexander ideal and does not depend on choice of presentation matrix. If "r > s", set the ideal equal to 0. If the Alexander ideal is principal, take a generator; this is called an Alexander polynomial of the knot. Since this is only unique up to multiplication by the Laurent monomial formula_4, one often fixes a particular unique form. Alexander's choice of normalization is to make the polynomial have a positive constant term.