On the basis of ab initio calculations of the electronic structure, an attempt is made to identify the reasons for the qualitatively different behaviours of long-period superstructures (LPS) with large and small superperiods 2M in alloys of noble metals. To this end, analysis is performed of the relationship between the nesting properties of the Fermi surface and the structure of LPS (of relatively small 2M) formed in the Ag₃Mg alloy. The results of the analysis are compared with those obtained earlier for Cu-Au alloys (where LPS possess longer 2M). It is shown that the dependence of the type of LPS on the value of 2M is controlled by the quality of nesting on the Fermi surface and the sensitivity of the nesting vector 2_F to variations in the long-range order parameter.