The role of a complete set of commuting operators (CSCO)is first recalled with the discussion of the electronic states of two finite systems as illustrativeexamples. It is then shown that its role isvery well transferable to sequences of finite systemsthat approach a real periodic system in the limit where the number of monomers becomes huge. In addition, the concept of the density of states (DOS) of total energy E, n(E−E0)(E0is the energy of the electronic ground state), is introducedas a system’s characteristic.