   Concerning the study of the expressive power of the combination of  fragments of classical logic mentioned above, both at the axiomatic  level, as well as considered as subalgebras of the 2-valued Boolean  algebra, Rautenberg obtained several interesting results in Rautenberg  1981. One of his main contributions was the proof that any 2-valued matrix  can be effectively axiomatized. In turn, the  question of combination  of fragments of classical logic by fibring was additionally investigated  in Caleiro et al 2019, based on the description of  the 2-valued clones made in Post 1941, on the axiomatization  procedures for 2-valued matrices introduced in Rautenberg 1981, and  on the results on fibring (N)matrices presented in Marcelino and  Caleiro 2017b (see subsection 4.3).  In particular, the  paper studied the conditions under which the fibring of the Hilbert calculi of disjoint fragments of classical logic recovers the union of the fragments.  Several examples of combinations by fibring of the logics of classical  connectives were analyzed, obtaining their characterizations by  (N)matrices, as well as the additional inference rules (bridge principles) needed  for recovering the corresponding fragment of classical logic. Among  several examples, it was shown that the fibring of two copies of the logic  of classical conjunction collapses into the logic of a single conjunction.  On the other hand, neither the combination of two copies of the logic of the  classical negation nor the combination of two copies of  the logic of the  classical disjunction produces an analogous collapse: the fibring of two  copies of the logic of  classical negation is not finite-valued, but can be characterized by a  5-valued Nmatrix; and the fibring of two copies of the logic of classical  disjunction cannot be characterized by a single finite-valued Nmatrix.  By analyzing once again the combination by fibring of the logic of  classical disjunction and the logic of classical conjunction (that is,  the least conservative extension of both logics) it was shown that the  combined logic cannot be characterized by a single finite-valued Nmatrix.  The same happens with the fibring of the logic of classical disjunction  and the logic of classical negation, thus showing that the fibred logics  are strictly subclassical in both cases.