The paper aims at studying, in full generality, logics defined by imposing a variable inclusion condition on a given logic . We prove that the description of the algebraic counterpart of the left variable inclusion companion of a given logic is related to the construction of Płonka sums of the matrix models of . This observation allows to obtain a Hilbert-style axiomatization of the logics of left variable inclusion, to describe the structure of their reduced models, and to locate them in the Leibniz hierarchy.