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The series connection linear time-invariant systems and that have minimal state space system descriptions is considered. From these descriptions strict system equivalent polynomial matrix system descriptions in the manner of Rosenbrock are derived. They are based on the factorization of the transfer matrix of the subsystems as a "ratio" of two right or left coprime polynomial matrices. They give rise to a simple polynomial matrix system description of the tandem connection . Theorem 1 states that for the complete controllability and observability of the state space system description of it is necessary and sufficient that certain "denominator" and "numerator" groups are coprime. Consequences for feedback systems are drawn in Corollary 1. The role of pole-zero cancellations is explained by Lemma 3 and Corollaries 2 and 3.